It’s funny, and Hollywood has done this forever. I have to give them a few conceits.
Firstly, getting actual teens is not just somewhat difficult, but it poses legal and other problems.
Secondly, teens by virtue will usually have less acting experience- less time to have acted. But they are also not viable long term as they are. What I mean, is that you would constantly need to find new teens because teens are only teens so long. Casts and crews often develop relationships and follow each other around. Star power and known names attract audiences and backers. And even if your star is a C lister or worse, any recognition can be good, and knowing they’ve been in the business and probably still will be later is a huge plus.
But there are some other very good reasons. Entertainment ones. Television paints in abstract and visual metaphor. The bulky bully or the hunky jock or the “sexy prom queen” need to be that for... everyone. Of any age that watches, if you want a wide audience. Television is often aspirational or escapist as well. Most kids self image leans more towards picturing themselves as this “adult” put together person. Most older people want to think of a fantasy version of themselves how ever many years ago.
I’m old. There are 21 year old celebrity “sex symbols” splayed out in enticing outfits and provocative poses on magazines and such- and to me... they look like children dolled up. Literally- like little kids. I don’t recall being able to find a 21 year old attractive for... probably at least 20 years, and even then It was case by case. I can’t take high drama and generally overblown depictions of serious life matters, watching kindergartners run around acting and speaking like adults.
The sick truth is that some 30,40+ year old is watching these “river dale” shows, maybe at obligation with a significant other, and the producers know that they need to provide “eye candy” that won’t make leches feel like criminals. A 17 year old “bombshell” played by a 17 year old girl- a grown man can’t tune in and ogle without being a sick criminal. Same for boys and women viewers. And there ar the moral, legal, and PR issues of specializing actual teens- but end of day- sex sells so they tend to pick “sexy” performers who can be legally and (questionably) ethically ogled by all ages.
That of course brings us to another issue we touched on already, but we will go a step deeper. Marketability. Yeah, if you search you could probably find men and women of adult age who have body types, faces, even height of an “average teen.” But if they become stars... what do you do with them? The “average teen” is a baby faced bean pole, awkwardly proportioned and generally not what magazines and products and movies want to hang their millions on. There are SOME celebrities, who tend to fill niche roles as Indy darlings and playing odd or skewed or off putting characters, often supporting roles, sometimes leads. But that’s not the norm. So if you’re developing a talent pool, you’re probably going to want to use people who have a better range and more mass appeal.
It’s a trope- a long standing one, that Hollywood uses people too old and too developed to play “average” teens. But I’ve seen very few works of fiction based on “teens” where they act, speak, etc. like “average teens.” They act and speak the way many teens want to think they do. The way some might actually, but by and large the majority don’t. Even teenagers get annoyed by other teens. We like kids when they are cute, funny, etc. the rest of the time, from baby to around late 20’s early 30’s... most people don’t really want to deal with those age groups being “authentic.”
But we don’t usually watch tv and such for fidelity to reality. At least not our own. Why would we? We live it already, we don’t have to watch what we live. We enjoy things that are close enough for us to project on, but different enough to offer us the things we want from life.
Firstly, getting actual teens is not just somewhat difficult, but it poses legal and other problems.
Secondly, teens by virtue will usually have less acting experience- less time to have acted. But they are also not viable long term as they are. What I mean, is that you would constantly need to find new teens because teens are only teens so long. Casts and crews often develop relationships and follow each other around. Star power and known names attract audiences and backers. And even if your star is a C lister or worse, any recognition can be good, and knowing they’ve been in the business and probably still will be later is a huge plus.