Well, my personal opinion..
FROM OLDEST TO YOUNGEST:
Applejack (Age 18)
Rarity (Age 17)
Twilight (Age 16)
Rainbow Dash (Age 15)
Fluttershy (Age 15)
Pinkie Pie (Age 14)
Spike (Age 10)
.:I know, Spike isn't part of the Mane 6, but hey, it says he's one of the 7 main characters:.