Sex is whether you are male or female (which sexual organs you have), gender is what/how you exhibit that. If you are male and would rather dress like/be with, etc. females, as one of them, you have a sexual disorder that revolves around your gender identification. It is controversial because it, as you point out, happens quite young and is probably similar to homosexuality, etc. in that that's not a disorder at all, is not a "choice".