One of the things I repeat the most, especially to teenage actors, is that in the acting business, you almost always want to look younger than you really are. Adults on set typically work 12 hour days. Because of the way child labor laws are, you cannot work those kind of hours if you are under 18. The law gets even more strict under 16, under 14, under 12… you get the idea. (This is why they often have twins playing the same child on TV shows – double the hours – but I digress.)
The point is, 99% of the time, they are not going to hire a 14 year old to play 18. They will, however, hire an 18 year old to play 14. Glee was a great example of this. Corey Montieth (who played Finn) was 29 years old when he died and he was still playing 19. He was in his mid 20s when the show started, playing 16.
Robbi here is a great example of what I mean. Looking at her, you’d guess she was 14, 15, maybe 16, but in fact she’s about to turn 20. With such a look, she could have some great success playing teens for years to come.