As mentioned, teenagers are bound by strict rules as regards schooling and what they are allowed to do on screen as far as nudity and violence.
This isn't a recent phenomena. It goes back to the silent days.
D.W. Griffith cast a 24 year old Lillian Gish in the role of a 12-year old child in Broken Blossoms (1919) because he believed no actual child was capable of playing the required strong dramatic scenes.
Most times, it is just box office power.
All the actors in Rebel Without A Cause were in their twenties, partly because James Dean and Natalie Wood were major stars and would be forgiven by audiences, even if they looked their ages.
Steve McQueen was 27 when he played a teenager in The Blob; Anita Corsaut was, I believe, about 30, and they looked it.
And This, Too, Shall Pass Away

do you know what a pain it is to work with teen actors… they STILL need set tutors and govt rules that will kill a production!!!!
it's not like it's easy to find ALL your teen actors who happen to be 18, the high school age…
The Fearmakers Blog
https://thefearmakers.blogspot.com/

for this one: there was nudity and parents had to sign off on their kid being involved in a film even with one shower scene.
others really because they know what it was like to BE a teenager and could remember how to act. a lot of kids who were stars when they were younger are completely fvked up now. Amanda bynes, Lindsay lohan, the Olson twins, Justin Bieber. Macaulay culkin very few child stars 'made it' into adulthood without some sort of mind blowing crap.
there is also the good chance the young adult actors playing teens could handle the role, behaviorwise. behavior as to how a teenager is supposed to behave/acceptable to behave and behavior as to how a young adult professional I supposed to behave onset.
and, with teen actors you need to pay for chaperones and possibly tutors. kaching kaching.
Oh God. Fortune
vomits
on my eiderdown once more.

At the beginning of "Coal Miner's Daughter", Sissy looked like she was in early teens. And that movie was four years after "Carrie".

They tended to look older back in the 70’s and Sissy Spacek could have passed for someone in their 30’s during certain scenes.
Kiss my black ass! Toodles!

Older actors are much better than teeny boppers and who wants to watch a teeny bopper movie.

this movie has the oldest looking high-schoolers ever

You don't need to watch a video to understand that most people in their 20s can still pass for a teen. When I was 24 / 25 I still looked like I was 18.