This age specific tag covers questions about kids after , but before they turn into teens, so say from about 11 years of age to about 14 years.
Rule of thumb: once they have been in school for quite awhile, but before they take a serious interest in the other gender (or their own, that detail is less applicable here than the romantic interest itself).
The moniker "tween" is sometimes also used, because they're between being a little kid and being a teen. The prototypical definition here is the tween girl: too old for toys, too young for boys. (It's just as applicable for boys, but "too young for girls", just dones't have the same ring to it.)