Since the State of New York passed legislation in October 2013, models under the age of 18 are considered child performers. So, they're subject to those same labor laws, which means they can't work crazy hours during NYFW (or ever, for that matter).
According to James Scully, who's played casting director for the likes of Jason Wu, Derek Lam, and Tom Ford, about 60% of models he's used in the past were underage. This season, however, he says just three out of the 350 girls he saw were under 18. "You know, they weren't just eighteen-year-olds, either," he told The New York Times. "A lot of these girls were really ranging between 20 and 24 years old, and I didn't find the aesthetic looked any different from when they were 15." Now that's one trend we hope becomes an industry standard. (The Fashion Law)