Teenagers in america are so adult-bodied as the movies shows?

[Millennium]

Kids are indeed developing faster; this much has been shown empirically. The current theory is that it might have to do with chemicals in most shampoo brands that mimic the effects of estrogen. However, the "kids" and "teenagers" seen in movies are still typically portrayed by people several years older than the character's supposed age. There are several reasons for this, but the biggest one is child-labor laws: if you can find someone who looks young enough to pass as a kid or teenager, they're legally allowed to work longer per day than actual kids of that age.