Some interesting ideas. I definitely agree with the central premise that the characters in the movie are supposed to be older than the characters in the book, but younger than the actors who are actually portraying them.

However, I can spot a couple of minor nitpicky problems with your estimates:

1. The school leaving age in the UK is 16, and yet Peter is depicted in school uniform at the start of Prince Caspian. Therefore the very oldest he is likely to be in this film is 16.
2. The age of conscription in the UK during World War 2 was 18, and yet Peter is not shown or mentioned to be in the Army during VDT. 

Potentially there's some wiggle room with both propositions above, where you could maybe argue about the technicalities of school uniform policies in.post-16 education in the UK, but by and large I would take those two factors above as key data points and shape the family ages around that. 

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I think though your estimates for Prince Caspian himself though are pretty spot-on. At the start of that film Miraz is ruling by proxy in some sort of Lord Protectorship role, implying Caspian is not yet of inheriting age.

I think it's a reasonable assumption to make that the age of adulthood in Telmarine society is 18, all things considered. 

FWIW, the early draft of the script for the first Narnia movie gives their ages as Peter (15), Susan (13), Edmund (11) and Lucy (8.) It's possible though that those were the ages the screenwriters were thinking for the actors rather than the characters. 

Chronicles_of_Narnia.pdf

@col-klink Oh! Interesting! So it really goes close to the ages I have suggested! Thank you for this!

@icarus Oh I didn't know that!! Well in this case I can only assume that Peter has struggled with identity after he returned to England, which is a topic I always discussed, not here but in general and he had trouble focusing on studies which meant he had to repeat some. Now I could say he could've been sent to war but this would mean that in the Prince Caspian movie he would be absolutely traumatized and PTSD would affect the plot a lot. I am going to stick to my own headcanon but these are very important reminders so thank you a lot! 

I am not bothered by character’s ages being a little different than those in the books as long as it is not any more than three or four years. It is true that Sophie Wilcox was a bit too old to play Lucy in the BBC series, but that is one thing that isn’t a great thing wrong with that series. They did get Lucy’s age right in the first Walden film, or appropriately right at least. What is much more noticeable is Ben Barnes age in Prince Caspian, where an adult somewhat inappropriately is playing a role of a child or a very young teenager. Barnes is the right age in Dawn Treader, and since he is a fairly good actor I wouldn’t criticize his role in that film too much.  That film is overly criticized, but I think Georgie Henley did a fairly good job when she returned, although she was a bit old for the part. She had the previous experience of playing Lucy in the first two films, and I don’t think anyone else could have played her.  She had the right personality for the role, which was the most important.

The casting in the Walden films was more professionally done than in the BBC series even though the actors weren’t exactly the same age as in the books. They did get their personalities right in most scenes of the films, and that is more important than exact age, although in some scenes the parts weren’t always played perfectly according to the books.

@icarus  The school leaving age in the UK is 16, and yet Peter is depicted in school uniform at the start of Prince Caspian. Therefore, the very oldest he is likely to be in this film is 16.

Not really. 14 or 15 would be an average minimum age for senior high school, but Peter could also be as old as 17, or even 18, depending on the school year. Back in the day when C.S. Lewis was writing, nobody was considered adult until they reached 21, in either UK or in Australia. 

There have been changes in the education system in UK since 1939 (and in related Commonwealth countries, Australia, in particular - well, I have good reason to know, after all.) Before World War II the minimum age for leaving school was lower than afterwards. My grandmother, for instance, left school at 12 years old to do piece work in a factory sweat shop before the outbreak of WW1. In those days, boys as young as 14 enlisted in the Army, when by WW2, the money paid to soldiers was so much better, for poorly educated boys on Struggle Street. Whilst my mother said she left school in 1939 at the age of 16 because her mother remarried.

My Dad was the proud owner of a good Intermediate certificate, having attended a private boarding school. But he didn't do nearly so well with his Leaving Certificate results, as I did in 1964 at an ordinary State School, that in UK would probably be the equivalent of a "Comprehensive" school. But by the end of that year when I left school, I was closer to my 17th birthday, and yes, all my classmates as well as myself, all wore school uniform until we left school. As was also the case in private schools, especially the sorts of historic private schools like, say, Newington or Kings, in Sydney, considered "Public Schools". 

A basic Intermediate certificate finished one's secondary school, but if one wanted university admission or a higher Matriculation qualification it was possible to stay at school for another 2 years. The minimum age for children to go to work was at least 15 years old, so 14-years-old students, having earned their Intermediate Certificate, sometimes were only marking time until their 15th birthdays came around, when if they wanted to do so, they could leave for work. 

The UK system was similar in some ways, only they use "O" levels and "A" levels. Whereas my Scottish-born husband, who left school at 15, migrated with his family to Australia, anyway. "O" levels are to indicate basic levels of competency in some subjects whilst "A" levels determine ability to cope with university work. In VDT, Peter was supposed to be staying with Professor Kirke because of undertaking higher studies.

Another complication in casting children is current labour laws, forbidding the employment of minors in exploitative roles. I'd say that also would impact the casting of children in the Narnia series. Gone are the old newspaper boys, doing a few rounds for a bob or two to help their mothers along. 

@waggawerewolf27 as you said, Leaving age in the 1930s was 14. My father did this.  However in families where their child was going on to university, there were further exams to pass, and both Lewis brothers spent time in live-in private coaching with a tutor - on whom the Professor was based. I'm away from my biography of Jack to check what age he did leave, but it might have been as young as 15.

Green and Hooper's biography of Lewis says that he began his studies with this tutor in September 1914.

So he must have been 15, two months before his 16th birthday.

The professor’s age in the BBC series and the first Walden film was one of the things that they got right in both of those productions.  I thought they did a good job in choosing actors that were very much like the books both in personality and appearance. It is encouraging when movies get something from the books right, and I wish they would do it more often. Perhaps it is unrealistic to expect that they will get every character’s age and appearance exactly right, but I like it better if the filmmakers at least try to imitate the original story. 

@coracle   However, in families where their child was going on to university, there were further exams to pass, and both Lewis brothers spent time in live-in private coaching with a tutor - on whom the Professor was based.

In C.S. Lewis' case I believe he was unhappy at the school he was sent to,  anyway, but back in his day, it was necessary to sit external exams to matriculate & then to pass university entrance exams. In New South Wales, the Intermediate certificate, another externally set exam, was usually good enough for most people before WW2. But after WW2 they streamlined the process somewhat, so that sitting the Leaving Certificate & passing well was all that was needed, provided the choice of subjects was suitable. Some subjects that girls often did such as sewing or cooking weren't considered suitable subjects for university studies, even though they were still examinable subjects. Of the total students who sat for the Leaving Certificate, at the school I attended in 1964, only a third were girls and boys predominated at the time.

Throughout one's school life, many of the exams are internal, set by the teacher, & based on the curriculum being studied. But major exams for university entrance or final school qualifications are often external exams, set by the State Board of Education, and the same for every candidate across the State. In PC, Peter was still only 14 in the book, & Prince Caspian was about the same age. 

Since Walden Media appears to be a Christian business they would likely get the characters of Narnia right including their ages. Douglas Gresham was involved with the movies at that time and he had some influence on the quality.  The only serious deviation from the correct age of the characters was Ben Barnes in Prince Caspian. Otherwise, the cast was chosen correctly with a difference of only a few years. I actually didn’t like the second film at first because it didn’t get Prince Caspian’s age right, but now I find it more watchable and more like the book than I had previously thought. For example, the scene of Aslan meeting Lucy was quite well done, and she is quite believable as is Liam Neeson as Aslan. Georgie Henley was the right age at the time, which was something that made the scene quite good.

Where did the name Walden Media come from?  Some say it likely came from Henry David Thoreau’s book Walden, which one would expect to produce films accurate to the books, including the ages of the characters. The bookstore chain, which no longer exists, also probably got its name from that work of literature.   Actually, I thought at first that they were connected, but reading into it I found that they were not related in any sense. I am sure that other people thought the same thing. 🙂

I tried a quick Search, and found a Walden University, and some uses as a place name. It may have been named after a founder of the company. 

Walden Media hired screenplay writers,  engaged a casting company, and everything else that's needed to make a movie of this sort. They were aiming at family movies, rather than Christian ones. Mr Gresham gave them a fairly free hand except if something was really wrong. 

Casting Caspian seems to have been difficult, and in the end Ben was chosen; he was currently playing a teenage schoolboy in a stage play at a leading theatre in London. Walden had to pay to have him released from his contract. He'd been a child actor, but was well into his 20s. He was meant to be a similar age to Peter, who by this film was being played as 16 (? He was playing 15 in the first film). Maybe both were playing 16-17. 

@coracle Casting Caspian seems to have been difficult, and in the end Ben was chosen; he was currently playing a teenage schoolboy in a stage play at a leading theatre in London. Walden had to pay to have him released from his contract.

I'd imagine so, especially as there are legal constraints now, at least in Australia on how young children can be when acting in major films. Recently, I heard about the cast of the BBC Narnia TV series being interviewed. They were telling about how they did the filming in the next studio to that used at the time by Jimmy Saville, and the efforts the staff went to, to protect their charges in the Narnia series, when already, people who worked at BBC, had an idea of what sort of person he was. These actors for the combined BBC/PC series, included:

• Richard Dempsey as Peter Pevensie
• Sophie Cook as Susan Pevensie
• Jonathan R. Scott as Edmund Pevensie
• Sophie Wilcox as Lucy Pevensie
• Jean Marc Perret as Prince Caspian (PC)
• Samuel West as King Caspian (VDT)
• David Thwaites as Eustace Scrubb
• Big Mick as Trumpkin
• George Claydon as Nikabrik
• Warwick Davis as Glimfeather and Reepicheep

The BBC series also used two different actors for Prince Caspian. Jean Marc Perret was replaced by Samuel West for VDT, but in the 3rd BBC volume, The Silver Chair, where Geoffrey Russell is listed as Old King Caspian, it was Jean Marc Perret who did the last scene of that movie. 

• Geoffrey Russell as King Caspian X
• Richard Henders as Prince Rilian
• David Thwaites as Eustace Scrubb
• Camilla Power as Jill Pole
• Big Mick as Trumpkin
• Warwick Davis as Glimfeather and Reepicheep

Whereas in Walden's films, Warwick Davis played Nikabrik, in Prince Caspian & Ben Barnes was retained for both PC & VDT, which was finally released two years later.

The BBC effort sort of worked, though Jean Marc Perret, allegedly the same age as Richard Dempsey, looked much younger, than Richard Dempsey's Peter, in my opinion. It would have made much more sense to cast Samuel West as King Caspian in that last chapter, at the end of Silver Chair, rather than Jean Marc Perret. 

@col_klink FWIW, the early draft of the script for the first Narnia movie gives their ages as Peter (15), Susan (13), Edmund (11) and Lucy (8.) It's possible though that those were the ages the screenwriters were thinking for the actors rather than the characters. 

I'd agree that the screenwriters were likely looking at minimum ages to get through the series. But I don't get an impression from the book that Lucy was much younger than Edmund. 

@narnian78 Georgie Henley was absolutely perfect for Lucy. For the record I don't think I would ever replace any actor in the movies at all.