I would agree that Georgie Henley was an excellent choice for Lucy.  I think that someone younger than Ben Barnes would have been better in Prince Caspian since Caspian was a young teenager in the book.  Ben Barnes was very good as the adult King Caspian in Voyage of the Dawn Treader. But casting people isn’t always so easy since the right people at the correct age aren’t always available.

@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. 

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. 

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. :)

@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. 

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

@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.