Usually it seems like no one there checks. Things are pretty unorganized sometimes. Same with chrono'ing, sometimes there isn't even a chrono there lol. So people usually set it in stone in the threads themselves, but when you get to the field, if you look 18 and just sign the papers normally, you could probably pass off for the whole day without someone noticing.
Really, the age seems to be 16+ for the players there. I don't think I've seen anyone playing at the games I've been at to be younger than that, from looks alone.