Personally, I think some aging up (key word being 'some') would be realistic - and would help the action aspect a little bit (some of Peter's heroics are a little more believable coming from a 15 or 16-year old than they are from a 13-year-old, personally).  Same with Susan's grown-up act.  And I agree w. Lucy, she simply doesn't work as a character if she's aged up (As BBC Narnia demonstrates).  Nor can Edmund be too much older than her either.

For what it's worth, I'll use my cousins as an example.  My one cousin is 8, and she coincidentally has 3 older siblings (who are 15, 13, and 11).  So, it is definitely doable, and in some cases would enhance the story (as long as that balance is there). 

I dislike ageing child characters up more than a year or two, and I equally dislike modern mid teenagers playing characters who are supposed to be 9 or 10.  Deep voices or womanly shapes are ridiculous, if the character is supposed to be a preadolescent.

Susan and Peter need to be older children, not young adults. The mass evacuation of children from London and other cities was for 'school age children', probably 3 to 13. The minimum school leaving age was 14 - when they could be useful and go to work.
Children going on to secondary schools were in a different situation; the Pevensies were clearly of the upper middle class who went to boarding schools, and would stay several years after 14, some headed for university and some for other training. Evacuation for these children might well be privately arranged, although Lewis never explains how this one happened (some script writers have suggested the Professor was a relation or a colleague of their father).

If the books are read and given to children from 5 or 6 years old, it is unfair to them to see filmed versions presenting people who are almost grown up, playing characters who are only a few years older than them. As a young reader I even thought the Famous Five original illustrations made the children look really old. [google those four words for examples]

Jill and Eustace must be children. Polly and Digory must be under 13.  And if the Pevensies are played by older actors, they must look no more than the Lewis timeline indicates.

I prefer kids the ages in the books for the reasons coracle mentioned above. But Will Moseley was 15-16 when he filmed LWW. I remember being very concerned about his age, but in LWW he was fine imho. Ben Barnes in PC was the one that REALLY bothered me, though I liked him that age in VDT. 

Honestly, I think the reason directors aim for older kids is they're more mature and easier to direct.  I will point out that Netflix hasn't shied away from using younger kids in the past. The kids in Stranger Things were 11-13 years old in the first season, though perhaps your point was they need to train kids in martial arts. But younger kids can be adept at learning some pretty hard things.

Posted by: @fantasia

I prefer kids the ages in the books for the reasons coracle mentioned above. But Will Moseley was 15-16 when he filmed LWW. I remember being very concerned about his age, but in LWW he was fine imho.

I just checked, and William Moseley was 17 when filming began for LWW in June 2004. For reference, Anna was fifteen and a half, Skandar was a few months away from 13, and Georgie was almost 9.

I totally agree about Ben Barnes, but if Walden's LWW is any indication, it looks like we need not panic if Netflix casts a 17 and 15 year old for Peter and Susan. I thought Will was 15 in LWW! I'm also reminded of the fact that Will Poulter was 16 when he played Eustace in VDT.

Oh, that's so interesting! I knew Will was 17 in 2004, playing 15 (IMDB says born April 1987), but I remember Anna saying she was playing 13-nearly-14, and was 14-nearly-15, so could remember 13. But IMDB says she was born in December 1988, so during filming in 2004 she was still 15.  Skandar turned 13 in September of 2004 filming, playing 11 or 12. Georgie turned 9 in July 2004, and was playing 8.

I hope Netflix can do things faster with its child actors, so they don't age speedily.
By 2007 filming of PC,Will was 20 (playing 16), Anna 18 (playing 15), Skandar 16 (playing 13), and Georgie 11-12 (playing 9-10). By 2009 Skandar was 18, Georgie 13-14, and neither of them could pass as young children, although we still loved and accepted them. 

[I just checked the spelling of 'Aging'; it's the US spelling, whereas the UK spelling is 'Ageing']

Posted by: @coracle

I hope Netflix can do things faster with its child actors, so they don't age speedily. 

In my humble (sometimes admittedly unpopular) opinion, that's another reason a TV series would be advantageous for adapting the first 4 books at least (in publication order - and maybe LB). 

I don't know that a TV series vs a film would make any difference, but @Glenwit makes a good point. It's far more important that they film back to back to back with minimal breaks in between to prevent aging, than it is to start young but have several years apart from each film/series. 

ETA: Looks like I need to credit @coracle as well for the above idea. Sorry I missed that!

I absolutely agree. 

And being Netflix, it's not uncommon for viewers to watch several episodes at a sitting, or even a whole series in a day!

Thing is, though, the characters DO age throughout the course of the series — and we have a pretty-much-canonical reference for this in Lewis's own timeline that he wrote after completing the books. The timeline does have some discrepancies and oddities in it, but overall it's a good guide. I'm hoping Netflix will be aware of it too and at least make the characters look about the age they're meant to be, even if the actual actors may be slightly older (hopefully not too much).

Here are the main characters' ages in each story, according to Lewis himself. I'm going in chronological order, rather than original publication order, since that makes more sense here...

THE MAGICIAN'S NEPHEW (set in Earth year 1900, Narnian year 1)

Digory (born 1888) - 12

Polly (born 1889) - 11

THE LION, THE WITCH AND THE WARDROBE (set in Earth year 1940, Narnian year 1000)

Peter (born 1927) - 13

Susan (born 1928) - 12

Edmund (born 1930) - 10

Lucy (born 1932) - 8

The Professor / Digory - 52 (see what I mean about discrepancies... that's hardly "a very old man"!!)

The Pevensies' ages when they leave Narnia, 15 Narnian years after their arrival

Peter - 28

Susan - 27

Edmund - 25

Lucy - 23

THE HORSE AND HIS BOY (set in Narnian year 1014)

Surprisingly, we're not given Shasta / Cor and Aravis's birth years in the timeline... I don't think we're given exact ages for them in the book either, but I can't quite remember. If anyone can correct me there, I'll edit them in. I always assume Shasta is about 13 and Aravis 12. The three Pevensies we see in this story — Susan, Edmund and Lucy — are one year off from their return to our world, so they're 26, 24 and 22 respectively.

PRINCE CASPIAN (set in Earth year 1941, Narnian year 2303)

Peter - 14

Susan - 13

Edmund - 11

Lucy - 9

Caspian (born Narnian year 2290) - 13

THE VOYAGE OF THE DAWN TREADER (set in Earth year 1942 (summer), Narnian year 2306-7)

Edmund - 12

Lucy - 10

Eustace (born 1933) - 9

Caspian - 16

THE SILVER CHAIR (set in Earth year 1942 (autumn), Narnian year 2356)

Eustace - 9

Jill (born 1933) - 9

Caspian - 66 (again, Lewis seems to have had odd ideas of what age "very old" is!!)

Rilian (born Narnian year 2325) - 31

THE LAST BATTLE (set in Earth year 1949, Narnian year 2555)

Tirian - age not given

Eustace - 16

Jill - 16

Peter - 22

Susan - 21

Edmund - 19

Lucy - 17

The Professor / Digory - 61

Polly - 60

I don't know why, but I always felt like Shasta and Aravis were slightly older than 13ish, more like 16. It probably didn't help that I had to read HHB at about age 12 for school. They (or at Aravis) seemed very mature to me at that age. 

Posted by: @courtenay

Surprisingly, we're not given Shasta / Cor and Aravis's birth years in the timeline... I don't think we're given exact ages for them in the book either, but I can't quite remember. If anyone can correct me there, I'll edit them in. I always assume Shasta is about 13 and Aravis 12.

I was rather thrown for a loop when you said that their ages don't appear in the timeline because I somehow just KNEW that they were fourteen, but now I had no idea why. Google to the rescue, which of course led me right back to NarniaWeb: Narnia Character Ages.

A footnote on that page says that Arsheesh discovered Shasta as an infant in the same year that the Tisroc came into power, and since that was also the year that the White Witch's winter ended, this means that (according to the timeline) Shasta is fourteen years old during the events of HHB. Another footnote suggests that Aravis is likely also fourteen because Shasta says he doesn't think she's any older than he is in Chapter 3.

I can see Shasta and Aravis as 15 or maybe even 16, @Lu_Valient, but imo, I think the story works best with 14 year olds... not quite children, but definitely not adults either. Still young enough that Bree and Hwin are very much the grown ups.

@courtenay I wonder if you knew that we have that timeline on here too: https://www.narniaweb.com/books/ages/

It is from Mr Hooper's Past Watchful Dragons, and from that source it also appears in Brian Sibley's book The Land of Narnia (I used to refer to this pre-Narniaweb, for discussions on the forum of Into the Wardrobe).

It doesn't, however, define the ages for Aravis, Shasta and Corin.  However, our timeline has a note explaining why we see the boys as 14, and Aravis as 13, based on internal evidence in the book. It's worth reading. ? 

I don't really feel that the books offer them much leeway in terms of altering the ages of the characters that much - at least if they are going to stick to the Wartime setting, and the idea that the children eventually become "too old" for Narnia.

For example, if Peter is supposed to be 13 in LWW, then the maximum age he can ever really be is 16 - a window of only 3 years, which in the grand scheme of things isn't all that much as far as I'm concerned.

I'm basing my logic here off the fact that if he were 16 in LWW, then he will be 17 in Prince Caspian. Both of these ages are young enough to ensure he wouldn't have been conscripted into  the British Army (at age 18) to fight in World War 2, and both of these ensure he is still technically a "child" during his Narnia adventures. Any older than 16 in LWW and both of these issues become problematic for the story tellers.

Posted by: @coracle

@courtenay I wonder if you knew that we have that timeline on here too: https://www.narniaweb.com/books/ages/

I didn't know that, or else I would have linked to it! Thank you for that.

It is from Mr Hooper's Past Watchful Dragons, and from that source it also appears in Brian Sibley's book The Land of Narnia (I used to refer to this pre-Narniaweb, for discussions on the forum of Into the Wardrobe).

I DID know that, having owned both those books since I was in primary school.  

It doesn't, however, define the ages for Aravis, Shasta and Corin.  However, our timeline has a note explaining why we see the boys as 14, and Aravis as 13, based on internal evidence in the book. It's worth reading. ? 

Yes, that is very helpful and makes sense — I see I wasn't far off in guessing their ages, but it's nice to know there is a definite way to date them.

I don't know how many viewers of Netflix (or any earlier screen versions) will be spending time debating over exactly what year this is set in or how old the characters are supposed to be — Lewis himself doesn't make any of this clear in the books themselves, only in the timeline, which wasn't published during his lifetime. I can think of at least one more discrepancy between the timeline and the books: in LWW we're told "there was only a year's difference" in age between Edmund and Lucy, but according to the timeline, it's two years. Unless Edmund was born very late in 1930 and Lucy very early in 1932, of course, so that the actual difference is only just over one year. But that's a minor detail.