Given Facebook's appalling track record with moderation I wouldn't want anyone under 13 (in my own care) near it. And that's before we've even gotten to privacy.
I might let my kids on HN - they'd probably learn something useful.
Certainly not reddit which one of the worse places on the internet for developing minds. It doesn't get mentioned much in discussions of "damaging social media" because it's anonymous and can't target people personally, but I'd wager that a lot more people have absorbed a lot more damaging content than just about any site I can think of.