I wouldn't worry about it, as has been said, there are other sites to work on. I can understand why a camsite might not want to have very young looking girls on their site (even if they are of legal age). Firstly because you will get the guys who want to fuck children/preteens. Some girls make really good money this way, but it would be understandable to not want this to be a part of the site. I know from some girls here who look a bit younger, even girls in their 30s, that they'll get those kinds of guys cropping up.
Secondly, gossip spreads. It's kind of like those guys who believe a girl has had a boob job because she's got out of proportion breasts, even if they are natural. They gossip, and rumours spread. Chaturbate couldn't exactly post proof that a girl is old enough without sharing her ID, so if rumours come up that they're letting underage girls cam, they may not want to risk tarnishing their reputation. They may just not want to deal with the busy body idiots who would likely report it.
To be honest I've seen plenty of camgirls on MFC who do look underage, and many who play on those factors and become successful, so you'd probably be fine there. Myfreecams is actually a bit of a creepy site like that if you think about it, a pretty popular personality is the skinny, small breasted "teen" girl with cuddly toys. Even if the girl is actually in her 20s, that "little girl" niche definitely seems like a popular one. I doubt the majority of their viewers are actually into the idea of "little girls", just like most guys with a school girl fantasy wouldn't literally want to fuck a school girl. It's just part of the fantasy.