"Younger" is a definition dependant on your own age. And I'm not the least surprised that the majority of the atheists are 15. I was on the bordline of atheist/agnostic when I was that age, there's a definitive apostatic slant in the 15-17 range, probably due to hormones and the feeling of ones own invulnerability.
I know what you mean. I was borderline atheist when I was their age. I questioned my faith for nearly 2 and a half years. In the end, when the hormones stopped controling me, when my hatred for authority died out, and when I basically just grew up, I remained Catholic, only now with a better understanding of my faith than I did before. In a way, going borderline atheist and questioning God and religion only ended up making me more religious, it gave me wisdom and and a new insight.
I think most of the kids here who claim to be atheists are just going through a phase. It's tied in closely with a disgust for any form of authority, cops, parents, schools, bosses, even God. Ah, they'll snap out of it, most of them anyway.