I don't know about you guys, but I always crack up when I see someone who is obviously over 25 trying to pass themselves off as being a high schooler in a movie or TV show. That being said, I had no idea that Olivia Newton John was 27 when she acted in
Grease.
Read about more full-grown adults who played angsty teenagers over at FlavorWire.
Link
Should be:
http://flavorwire.com/198664/some-of-films-oldest-high-schoolers