I'd go with 15-21. I realize this is alienating a whole lot of you guys, but 13 year olds still have a lot left to learn and I feel like the forum might be a little less drama based with the average user's age shifted up by a solid 2-3 years. That said, these wouldn't be strict restraints. I wouldn't be unhappy with 13 year olds or 23 year olds hanging around, just I'd rather have neither be the average.