It is more common with children. In my opinion, children haven't formed the notion of "reality" that most adults have. Their natural curiosity makes them prime subjects for becoming lucid. Also, as people grow up certain things become higher on a list of importance and those things are focused on. Naturally, dreaming becomes a lesser priority and so we begin to lose recall and, for many, our questioning nature that is key to LDing.

That's just my take on the issue.

-Amé