Because they’re not really Christians?
I think that there are a bunch of kids who become Christians early, and at some point they question and rebel from God and Christianity. They might not do it overtly, but in little ways like sex, drinking, etc.
I think I did that when I went to college. As I grew up, I understood more, and got much stronger in my faith. Plus, I had events in my life that tested my faith. The biggest was when my twin babies almost died. I think God got me through that, and I have been much stronger in my faith since then.