Twilight Girl

I think that Fluttershy is the eldest at 18, as when they all get their cutie marks she looks quite a bit older than the other 5, which would make Pinkie 17, because in the episode Griffin The Brush Off Fluttershy tells Pinkie that she's a year older than her. I would then say Rarity is 17 and a half, one because she has her own business and two because she's roughly the same sort of age as Fluttershy and Cheerilee. I would then put Apple Jack at around 16, she's only helping with the whole families business which Big Mac runs, and plus she still seems to be learning, especially when her pride gets in her way. RD is probably older than Twilight, because Twilight is still studying, at 14/15. Last is Twilight who I would put last at about 12/13, because she still seems very unsure about things and is still in the process of learning. My last thoughts are this: Fluttershy (18), Rarity (17 and a half), Pinkie Pie (17), Apple Jack (16), Rainbow Dash (14/15) and finally Twilight (12/13).