Exactly; adults playing teenagers. However, I will say that teenagers dress way more adult than in the past.

Its because we teens are pressured into looking as good as an actor, we want a six pack.

I actually think a lot of this 'kids are being pressured to grow up' stuff is bull, society is expecting people to remain children when they are not children a 15 year old is not a child, they are a teenager and should be expected to act like a teenager.

/Rant

c'mon guys, there is a WAY more practical answer to this:

child labor.

Teenagers who are 14-17 still have to go through school and thus their work schedule is limited. I'm not sure exactly how much, but being forced to stop working after 4 hours is not uncommon.

Mark Kate and Ashley Olsen got big mainly because they were twins-- one kid could work 4 hours and the other kid could work the other 4 hours, thus being able to do a full work schedule.