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So first off there are plenty of shows already about people at pretty much any age group, and in any of those shows you may very well have a 38 year old playing 30, or a 27 year old playing 35, it doesnt really matter. 
With teen shows, there are laws limiting how long, and what hours, a minor can work, as well as needing a guardian present to make certain decisions. 
So if I hire a 20 year old, they can work 12 hour days. We can film a night scene at night, it might take 6 hours to shoot, so we can shoot from 9pm-3am, which we can't do with a 15 year old.
The other issue with actual teenagers is it can be quite difficult to predict how they are going to physically develop. So if you go away for 4 months for summer break, all of a sudden when you come back and one of your stars has changed, you may have to retool the dynamics of your show that you've spent 3 seasons developing, or actually throw out some scripts that the writers spent weeks or months working on. The likelihood of a 22 year old developing physically in a short time span, is pretty low. 
Also, I think many adults watch shows about teens, they are not geared specifically towards teens. Typically you want as broad an appeal as possible if you want to keep a show running. The 25 or 35, etc. year olds are probably not going to watch a show with 15 year old leads, where as even the teens are just as likely, even more so maybe,  to watch a show with 20 something leads.
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Have you considered that most adults are working and teenagers have more screen time, so catering to their needs is more profitable? >

I actually think people in their twenties have more time to watch tv, high schoolers (atleast in my country) have to study all the time (even on weekends)
I can say for myself that I have way more free time at age 20 then I did in high school >

Do you think the average 16(mid teens) year old has less screen time than an average early 20s person? >

Yes >

Are you from an upper middle class family... >

God damn you called him out hard >

Young adults are at the time in their lives when they are most busy. Collage, university, trade schools, starting work and often working as much as they can. Pre-covid at least busy social lives. They just don't have the same amount of time to spend watching tv as teenagers do. >

i had so much more free time in college