I don't know, but probably at least through High school.
Yes, I quit monitoriing my older son when he was about sixteen.
well, my 14 YO daughter isn't immune from seeing crude, raunchy stuff over there either.
I don't think that it's wrong to friend minors, but I do think that, as with any interaction with other people's children, there is a certain amount of risk involved for which one should be prepared.
The Washington Post had a great article about communication with your college student today:
of the "minor" friends i am friends with there i am also friends with their mothers, there was one friend of my daughter's whose request i accepted knowing that her mother did NOT have an account and i was a little leery about that,