I think they’re beetween 16-18. It could be that they aren’t all in the same class. Because in "Last day of school" Sci-Twi and RD are in the same class, so they’re the same age and juniors. Also in "FOMO" we see Rainbow Dash driving a motorcycle, so she's 16-17. Sci-Twi doesn’t drive a car, either, because in "Friendship Games" and "Monday Blues" she takes the bus. So she’s the same age as Rainbow.
Fluttershy is older, I think. In "Driving Miss Shimmer" she has her own car, so she’s obliviously 18. Sunset Shimmer, too. Also Rarity and AJ.
About Pinkie Pie, I really don’t know. I think she's 17, because we all know Fluttershy is a year older than her. Also I haven't seen her drive a car or something.