4-5 years ago I RP-d on another site, of course using forum format and all.
It was a fantasy/superhero like contemporary setting.
I have many good memories from that time and a number of good players. There was a particular new player who was decent with her characterisation and IIRCwon the best RP-er title in the site's unofficial awards on the same year.
She later told she was 13.
So yeah, I have no reason to think youths cannot join RPs.

But that doesn't give you an excuse to feature them in a game which is strictly 18+.
Whether it'd expose him/her to something inappropriate or not.
It's the rule and above that, all the other players are assured that everybody is an adult.
Likes others said this would be betraying their trust.