amagical hand wave and Aslan's paw. Seriously. I have them at 16 and 15, 13 and 12 in the summer of 1942 because that's how old I wanted them to be and I backed up from there. The implication is that both Peter and Susan have early birthdays in 1943 -- Feb, March or April, but I don't specify.

In short, I made the ages I wanted them to be and stopped worrying about it. The only complaints I get about it are when Peter is off to war in Maenad of the Maquis -- and yep, I know he's too young to be a flier in the summer of 1943.