Maturity is the one that is needed to be judge, unfortunately we can't base on every internet function to identify that. The real mature kiddos won't let anyone have the knowledge of them being under-age, thus some youngsters on internet had the ability, take an example, digital arts. Besides, we're all humans and the only difference being youngsters to seniors are basically maturity change and sorts. Even the senior can be a bunch of immature dudes; (Example, rabid bronies, weird creepypasta fangirls, annoying homestuck spam).
But yes, maybe age below 9 can be a bit risky since sexual and violence being feature on this page. 10 and above? It's the fact of maturity since themselves must've known at least sexual reproduction and going through puberty, with talents to express. 13+ and above is reliable, but to be honest, it's just the same as being 10 and above, I guess.