>>29353609
Femboy is a just a more granular descriptor. A sissy or a crossdresser can be any man from the tiniest hairless young twink to a hairy 50 year old with a beer belly. As long they crossdress they are a crossdresser or a sissy.

Femboy refers specifically to young, petite boys with feminine features. It's basically a word for the peak of the Twink form. Someone above the age of 30 or even 25 can't really be one unless they have insane genetics or micro dose estrogen.