[Jamie89]

I suppose it depends why they're doing it, as to why they don't dress "appropriate to their age" etc. If they're doing it for fun, to be experimental, for a sexual thrill etc then I suppose that might be why, if they're not necessarily trying to pass themselves off as a female version of themselves, then why be restricted by norms of what society deems to be appropriate.

It's only really transexuals who would generally want to wear "appropriate" clothing in order to fit in, which in the vast majority of cases they do (obviously there are some who don't but this is also true of non-transexuals).

So I suppose to counter your question, why shouldn't they?