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Problem is with casting actual kid actors is they aren't done growing which becomes problematic unless your show is doing time jumps.  Look at Stranger Things where only 2-3 yrs total have elapsed in show time but all the kid actors aged eight years and show it.
The reverse is kinda funny though.  In House of the Dragon they cast a then 20 yr old to play the 14 yr old main character.  Then mid season do a time jump and recast her with a then 28 yr old actress.  Which I just find humorous since the original actress was actually closer to this second age than the one she played.  Why not just keep using her and make her look older with makeup instead of younger.
Kid actors usually suck and it's usually some parent-influenced career choice that they won't continue into adulthood anyway. I'll take a decent 20 year old over a 15 year old almost every time. >

Yeah unfortunately the problem is that the shows that feature those "teenagers" are targeted at actual teenagers and it pretty much warps their entire reality of what being a teenager should look like.