Why are older actors playing younger roles?

So I was watching The Hot Chick the other night.  I noticed one of the actors in it playing a high school student also played one in 2009’s Fired Up!.  So I checked to see how old The Hot Chick was.  It turns out it came out in 2002.  Not really surprising the actor, Eric Christian Olsen, is still playing younger characters, as he does look young, for his age.  

Old actors playing teenagers

Not only does it turn out he was way over the age he played in Fired Up!, but he was also much older than a high school student, at least one not held back constantly, at the time The Hot Chick was filmed.  He is close to 34 years of age.  His costar in Fired Up!? Well he, Nicholas D'Agosto, is about 30 years old.  Kind of ironic they were cracking jokes about people over 30 in the movie huh? Good inside jokes from Hollywood I suppose or perhaps just simple irony.

John Hughes was credited with having real teenagers in his movies.  That is true to a point.  Anthony Michael Hall, Molly Ringwald, Mia Sarah, Ilan Mitchell-Smith were all teenagers. Most of us know Matthew Broderick and Alan Ruck were well beyond his teenage years by the time Ferris Bueller's Day Off came out though.  This goes for many other stars of his films.  

Why does Hollywood constantly use older people to play teenagers? Someone suggested perhaps child labor laws, or because they want scenes that require nudity, kissing, etc that child actors would not be able to perform.  

Could it also just be that Hollywood wants everyone to have a perfect image? Lord forbid a teenager in a movie have a blemish!

So do you feel since movies are fantasy Hollywood should continue to put the best possible actor/actress, regardless of age, into such roles, or should they hire more true to age? Thoughts?