In many films, I have seen that a lot of teenage characters are played by actors in their 20's.
Two notable examples are:
Why were older performers cast in these roles? Were actual teenagers deemed unworthy?
There can be various reasons such as dealing with adults is more manageable than with teenagers, child labor law, guardian interface, etc.
Marcia Ross — who cast 10 Things I Hate About You and Clueless — explains, in most cases, “actors under the age of 18 need to have guardians on set and work a limited set of hours because of schooling.” So, casting older actors becomes more of a financial and production decision. Though, for both of those films, she clarifies that there was no mandate about what age the actors should be. - teenvogue
Also, you can find more adult-trained actors who can be easier to work with rather than inexperienced teenagers who might.
Also, teenagers go through changes more drastically in less time interval
“They do tend to grow up a lot,” he told us at a charity party in North London. “In one film they were children but when I saw them again they were teenagers. Now they are adults. I still talk to them as if they were children and I have to stop myself.” - Jason (From Express)
Adults, on the other hand, have more experiences, have fewer bodily changes, and no worry about the limitation of child labor.
It's a common trope and called Dawson Casting
