Why in alot of american movies actors are cast to play characters with younger age than in real life?

Especially in high school movies where up to 25 yrs old actors look ridiculous as high school boyz which suppose to be maximum 18.|||The worst was Stockard Channing in Grease. She was 35, more than twice the age of her character.





Also Glenn Close in South Pacific, more than twice the age of her character, although character wasn't a teenager.





Basically child labor laws make it hard to have children starring in movies, and adults are more likely to be experienced actors and box office draws.|||Coz americans are retaarded|||they probably are better actors than a younger random person|||They think we are stupid.