21 Jump Street pokes fun at this concept by having the characters be 30 somethings pretending to be teenagers, but even the teenage characters are played by 20 somethings.
Shooting a film with actual 16 year olds is hard. Usually they’re only available for a limited time because they have to go to school. Even if they’re home-schooled, labor laws are very strict about how long children can remain on set each day. And as a general rule 16 year old actors aren’t even that good because they have so little experience.
Channing Tatum was 32 in this scene, a flashback to his character’s actual time as a student.
Typically instead of hiring teenagers, movies and TV shows will hire actors in their 20s. They’re easier to work with. More skilled as a rule. And it’s done so frequently that we don’t even notice it any more.