It would be interesting to look at those nations, such as Israel, where more women become programmers, and look to see if an early start is a factor there. I would guess that institutional forces are at work at a later age, in particular the willingness of the schools and the military to push math/computers to anyone who seems to have ability. Which is to say, I doubt that starting early is a factor. What matters is what sort of environment the women face when they get done with school.