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 to.
Ally Sheedy does not have a birthmark on her face as Jaimie Alexander does, and the photos of "young Sarah" show the birthmark. Makeup could have fixed this problem going in either direction.
All of Kyles abilities are explained by him using 97% of his brain rather than the 7% that the average human uses, however humans only using 7% of their brain is a myth. Every human uses 100% of their brain at all times.