Writer's Block: Old enough!?!

What do you think the lower age limit should be for LiveJournal and other social networks, and why?

I don't think there should be one fixed limit for all social networks and multi-player games.  I think that parents should help young children who are interested enjoy age-appropriate spaces on line,  and I think that social-networking forums should make available different types of accounts with different privileges, say one kind suitable for people under 12, and another kind suitable for 12-14 or so.  It could still be Facebook, but with enhanced privacy defaults in both directions, no ads, and a block on using payments in games.  Or it could still be Livejournal, just without profile information and with blocks on not-safe-for-kids content.