According to this timeline, Peter was born in 1927, Susan in 1928, Edmund in 1930, and Lucy in 1932. They traveled to Narnia for the first time through the wardrobe in 1940, at the respective ages of thirteen, twelve, ten, and eight (and spent fifteen Narnian years there, if you were wondering). Their return to Narnia at the summons of the horn occurred one year later.
And the actors and actresse? During filming for the two movies, the actors were, respectively and approximately, sixteen and nineteen (Peter), sixteen and eighteen (Susan), thirteen and fifteen (Edmund), and nine and twelve (Lucy).
Not great, especially the older Pevensies...but I can deal with it. What I can't deal with is this: Prince Caspian X was thirteen years old at the time of the war with Miraz. Ben Barnes, the actor who played him, was twenty-five at the time of Prince Caspian's filming. :beating forehead with wrists: Is it any wonder Will kept quarrelling with him? I'd be testy too if I had to deal with that casting decision on a daily basis.
On another Narnian note, don't you think this model would make an excellent White Witch when she got to be older? Just the right kind of otherworldly freaky-deaky, in my opinion.
