Re: I did a new movie-related piece for RetroJunk...
I would argue that's it's becoming somewhat a thing of the past. From the beginning of film until the 90s teenage characters were almost always played by someone older. The obvious reason was to avoid the difficulties of working with a minor (legal guardians, child labor laws, etc.). Plus, since we almost always saw twenty-somethings playing teens, it began to look almost normal to most audiences. Many times I have heard people remark how young high-school kids looked in real life instead of remarking how old they looked on 90210.
In the mid-nineties we finally began to see a shift. Maybe it was because audiences were becoming more sophisticated, more likely it was because studios didn't want yet another 30-something appearing in the final seasons of a 10-season long teen drama; but Hollywood began to cast teens playing close to their own age on a regular basis. Regardless of the reason at the time, it is now an expectation. I doubt modern film audiences would accept an actor playing a part too many years too young anymore*.
If Harry Potter had been a series in the 50s-80s we would have certainly seen the characters played by actors 18 and up, but now they could never get away with it. Heck, I'm still impressed with the fact that they were willing to cast a 17-year-old minor for the role of Peter Pevensie in Narnia instead of an 18-year-old. (William Moseley didn't turn 18 until the movie was about to be released.)
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* One exception is TV, specifically UPN/WB/UPN teen shows. Studios do not want to deal with child labor laws on a weekly series and luckily the audiences for these shoes don't care that the stars are obviously too old as long as they look hot enough.
