Wrong aged actors?

k over the past few years i've noticed a great increase of how 15 to 18 year olds are being played by like 25 year olds. like in Transformers, shia lebouf plays a 16 year old and he's like 21...he soooo doesn't look 16...same thing with the upcoming Narnia movie, high school musical, hairspray, superbad(awesome movie!), harry potter and waaaay more! these people dont even look like the ages their playing...so whats the point of even trying? why are directors doing this? WHY??