i have a fb account. i think im more stringent than most. (my son is 5 so hasn't had to worry about this yet, but i'm anticipating the day...-although a coworker's 5 year old has one - i don't think she "uses" it, more of a "fan" page...don't get me started...)
of course all my "stuff" is private - friends only. and i only have about 100 friends. that seems like a lot to me but it is all personal friends (not "acquaintances") and family. i have several classmates from hs that i have reconnected with. but i have friended many acquaintances and "old" friends that turned out, are kind of dirtbags, so out they went. also goes for people who never, ever, ever say a word to me on fb (or in rl either), or update their status. that just creeps me out. why have it? delete.
i also don't post pictures i wouldn't want my grandmother to see - she is a friend on my fb. as are great aunts and uncles, cousins, etc, from all over the country. i definitely keep it family friendly.
my child will have to give me their password and while i won't insist on being their "friend", i will keep tabs on what's going on. nothing his grandmother wouldn't be able to see should go on his, either.
there's no privacy on those things. there's only the illusion of privacy. anyone who posts party pics, drunk pics, smoking (*whatever*) pics...just looks like a not-so-intelligent person, imho.
i don't know when i'd let him. honestly i don't even know what the "legal" age is. he'd have to show interest first, which he doesn't. then i'd have to think about it. i'm not sure the "legal" age is what i would go by strictly...especially considering i would set it up, i would have the password, etc. but i'd have to feel comfortable with whatever might pop up, so i would have to think about that. hopefully it's years away! :)