Why do they mostly use actors in their late 20s or early 30s to play teenage roles in movies/ tv series? Couldn’t they use teenage actors instead?
Underage actors have more restrictions on their working schedules due to labor laws, usually a limited number of hours to film each day. Full grown adults don’t have such restrictions, so it’s easier to schedule longer filming shoots with them.
it’s easier to get an experienced actor to play a teenager than it is for a teenager to act like an experienced actor
There’s a few downsides to using actual teenagers.
First off, they’re usually less experienced so it’s more difficult to find quality teen actors. The ones in their twenties have had more chance to practice, develop and prove themselves.
There’s also child labour laws. I’m not sure if this still applies, but back when Harry Potter was filmed they had to use doubles for the main cast in background scenes. The work limits for minors make the filling take too long otherwise. Having an adult actor means having someone who can work long days and speed up filming
Logistics. TV series have it (slightly) easier since they are mostly filmed on set at a fixed location. Recent “mini-series” TV shows might not have it as easy. Movies, though, tend to be filmed on locations and usually many locations. For younger (non-adult) actors, there typically needs to be a guardian and close supervision. Some shoots take many hours and child actors are legally not allowed to work as many hours as adult actors. (in most countries – there are specific labor laws covering not yet adult workers)
For the late-teen characters, the difference isn’t really that huge and the looks tend to carry the scene. It isn’t difficult for a young looking 20 something to look a few years younger. And it avoids all the logistics as above.
Movies tend to be expensive to produce. Delays cost money (could be tens to hundreds of thousand dollars a day for even a modest production) and there is always a bit more risk hiring an unproven or less experienced actor over someone with 5-6 years experience. The skill levels (broadly speaking) are not comparable.
And, of course, there is star power. Top actors (even young) are expected to draw in sales.
Of course, these are broad statements and there are certainly exceptions.
It’s a combination of two big factors. First, there are laws about how much an actual teenager is allowed to be working versus going to school. It is much more difficult to film a TV show where you have a time limit on how many hours a day the teenager can work. So in that situation, it’s easier to get an older person who looks young to play a teenage part, because they won’t have the same restrictions and regulations that an actual teenager would.
The second is the belief that a young adult actor will have more experience and be easier to work with than a literal teenager.