In another thread, I posted a Narnia timeline I pieced together based on details from canon; see below. The "official" time line definitely conflicts with the books, and it might not have been written by Lewis at all, since it wasn't discovered until after his death.

1927-Peter is born
1928-Susan is born
1930-Edmund is born
Late in the year 1931-Lucy is born
Somewhere between 1934 and 1937-Jill and Eustace are born
1944, July-events of LWW
1945, Sept-events of PC
1946, summer-events of VDT
1946, Oct-events of SC
1948, April-events of LB

According to this, Peter and Susan are in their mid teens in LWW. William and Anna actually were in their mid teens when they filmed LWW, the perfect age for Peter and Susan. Georgie was a bit young for the role, but then I don't think it killed the movie.

Actually, based on my math Georgie was about right on target age-wise, Skandlar was 2 or 3 years older than his book counter part, and Will and Anna were 4 or 5 years older. (The bombing of London was in 1940 & 1941, so that's when LWW started. [The movie at least, the book might be a bit more lenient, but there's really no mention of a date that I know of except that they were sent away because of the blitz, or the timeline that may or may not be official.])

I believe partly this was so the kids would look more alike, and I'm guessing partly because it's sort of a "coming of age" story for Peter, and now kids aren't really considered of age until they hit their late teens/ early twenties. I will note though Will and Anna both looked younger than their actual age in the movie, in my opinion.

Then of coarse they cast Caspian and Eustace older to match the ages of the aging Pevensie actors.

When I read the books, I've always had the kids ages in LWW as:

Lucy 8 or 9
Edmund 9 or 10
Susan 10 or 11
Peter 11 or 12

And they've probably been those ages in my mind because they are referred to as children.

And Caspian was also too old in the movies - he also should have still been a child.

Also, when I'm reading the books, I get the impression that Eustace is the same age as Edmund - simply because he refers to Lucy as "just a kid after all."

I've never looked at the timeline before, but it just doesn't seem to go with what I read in the books. And no matter what anyone else says - these characters have been living in my head for a long, long time - and they're ages are not going to change for me. I wouldn't expect whatever age you have imagined them to be to change.

That's a very good point - there were no teenagers until the 1950s started the vogue of the "teen". You were a child until you left school!

(The bombing of London was in 1940 & 1941, so that's when LWW started.

Re the book timelines: The doodlebug bombings and evacuations happened in 1944. Also, in VDT, which happens two years later in Earth time, it is implied that the war is already over. So, it would make more sense for the Pevensies to be a part of the 1944 evacuations if we are discussing the books.

For the Walden movie timeline, the earlier bombings make more sense, but it doesn't affect the children's ages in the books.

Here is one Wikipedia account of the WW2 evacuations, and the BBC has more. There may have been more evacuations when the Doodlebugs were sent against London, but there were preparations to protect children from the last day of August before WW2 started on 3rd September, 1939.

LWW may well start at this time, though, since evacuating children and other civilians continued at other times during that war. At first in 1939 there is no initial threat in what was the phony war, but the wisdom of the move was more than vindicated when the Germans stopped bombing only airfields, and started bombing London the following September, in the Battle of Britain. However, UK resistance, reinforced by Commonwealth, Polish and Czech pilots, seems to have halted Operation Sea Lion, the projected German invasion of UK. It wasn't only to the countryside these children were sent, but also to Canada, and at least one ship taking those UK children there was sunk in the Atlantic.

I don't know why exactly the children's ages were changed when filming the Narnia series, apart from the obvious practicalities faced by the film directors in keeping the same actors over a period of time. But C.S.Lewis, who wrote these stories after the war, having based some of the characters on the childish play and attitudes of the evacuees he hosted, probably wasn't really wanting to go into WW2 specifics and events in the entire series, anyway. The very historic 1939 evacuation was undoubtedly a good starting point. There are other, non-Narnian children's books which depict those times as well, such as Michelle Magorian's Goodnight Mr Tom.