Hunter Schafer never really intended to be on the show. Her modelling agency saw the casting call and hardly insisted for her to audition, suggesting that she would be great for the part.
I'm envious of your generation, you know. You guys don't care as much about the rules.
Throughout out the series, it's clear that the actors/actresses portraying teenagers are actually in their 20s+. This is done as to not interfere with the schooling of real teenagers and to give the general viewing audience who are teenagers themselves someone they can look up too.