Why are most young animated characters voiced by women?

Why are most young animated characters voiced by women? This is notably evident to Bart Simpson, a fictional animated character where he is a 10 year old boy yet he is voiced by Nancy Cartwright, a woman. Is it because men have a too deep voice? It would seem so but why couldn't children voice them since they are a closer age? This happened to Bambi but it is quite odd for women to voice children. Why does this happen?