Well, it all depends. Yes, realistically, I want someone who looks/sounds like a teenager and, while I think it'd be entertaining to see Anthony Hopkins do a London Teenager (in voice), I don't believe he could physically carry it off. Though they are doing interesting things with digital effects nowadays (The "young" RDJ in Captain America: Civil War was pretty well done, the "young" Michael Douglas in Ant-Man wasn't bad...)
That said, there are plenty of examples of actors pulling off playing younger and you don't necessarily notice it. Heck, lots of "High School" roles are played by people over 18 (child labor laws being what they are).
I'm reminded of Goldie Hawn's line in First Wive's Cliub: There are only three ages for women in Hollywood - Babe, District Attorney, and Driving Miss Daisy. Though I'd argue it's "Assistant District Attorney" (can't have a woman as a DA in Hollywood...)