A fun game is to Google the ages of the actors who play teenagers on whatever television show you're watching. Teen Wolf, for instance, provides a pretty good case study: Tyler Posey and Dylan O'Brien, who play seventeen year olds, are twenty one, which is bad but not so bad. Crystal Reed, who plays Posey's seventeen year old girlfriend, is twenty eight years old, which makes her technically senior to Tyler Hoechlin, whose character, Derek, is somewhere in his early twenties. (Hoechlin is the only actor playing within two years of his actual age.) The show also features the queen of this phenomenon, Bianca Lawson, who first played a seventeen year old character in 1993 and is still doing it twenty years later-- though her Teen Wolf role is as a guidance counselor and adult. Twisted does better, but not by too much: Jo, Lacey and Danny are supposed to be seventeen, and they're played by actors who are 18, 24 and 21, respectively.
It makes sense: the blog Actual Adult, Actual Teen is your one-stop shop for remembering exactly how dweeby and reedy teenagers (especially teenage boys) really look. Adults have clearer skin and bigger, um, biceps and things, and actors who are over eighteen can work longer hours and don't require on-set tutors. Teenagers are young and dumb and full of possibility; these characters can get away with behavior (and drama levels) that would be difficult to watch coming from characters we were meant to understand as adults. And so television shows get the best of both worlds, bringing in maximally beautiful people to act out maximally emotional stories, a world of cathartic, escapist melodrama inhabited by people long past the awkward stages of their growth spurts.
There's plenty to be said about the way this affects us as individuals and as a culture, the way that is makes teenage life seem fun and sexy in a way it really, really isn't, and ages up our understanding of what it means to be, say, a seventeen year old girl. (Not to mention what it does to seventeen year old girls, who look at the women on screen and can't understand what their hair and skin and bodies don't look like that-- not understanding that they don't look that way yet.) It's fun and easy, but, like most things that are easy to swallow, damaging in the long term. We're a culture obsessed with youth, yes, but only with the pleasant parts: we mine the misunderstandings and heartbreaks because a sad middle gives shine to the happy ending, and anyone who actually look young and vulnerable makes that too real for us to take.