Age limit? Are you fucking kidding me? As if people don't lie about their age ALL THE TIME. Myself, my friends, and everyone I know has been on social networking sites since about 12, and no harm has come from it. I'm 17 now, and I'd say age limits are useless. No matter what limits or expectations you put on a site, there are always going to be the same number of trolls, hackers, annoying people, and mature users. People tend to find a way around things.
The only harm that I've seen come from it is when my sister was about 15-16 or so, she fell for some person on myspace who wasnt quite who they seemed to be. It only went so far as her flying somewhere to meet him and he never showed. And that was because her mother let her go there, and let it all go down. Same mother who gave her a hotel room and alcohol for her and her friends on her 17th? birthday. So.... I think that says it there.
It's really all about the individuals maturity. Bottom line. And parents having better communication with their children. If the parent wants to block the site, that is their deal, but it shouldn't be up to the internet. Nothing should.