18 and up is a pretty universal age-line. The only reason this enters the realm of site policy, at all, is protecting the Guild and those who run it. Except it's not, and it shouldn't be: every rule from the Fonz to this smut-section-age-restriction is in some way representative of the Guild and those behind it.

I feel confident in saying even a purely academic discussion on age limits lower than 18 ignores the fact that there are very real, very creepy, people out there. (And neverminds that even most 18 year olds don't have their lives, let alone their sexuality, well put together.) And as members of the Guild, we'd like to protect our younger members as much as we possibly can. Sure, because we want to not be charged for some Federal crime, but also because anything else just makes us uncomfortable as human beings. 

18 and up seems to fit the spirit of the Guild well enough, though I'm sure there are those out there who'd like it to be younger, just as I'd be okay with it being older still.