I'm British, so for me it's 16. But LJ is operating under American laws, which are problematic. It's not inconceivable that some nutcase could file a suit against them on the grounds that they're allowing access to sexual content to minors under the age of 21, given some of the state laws and "community standards" rulings out there.

They do have a two-step system, with one tag blocking access to people under 14, and another blocking access to under 18, which I think is a good idea. *If* it's simply a tool for people who'd like to control access to their own journals, not if it becomes a tool for people to block access to anything they disapprove of.