As I said before (on another forum):
Rarity: 20
Applejack: 19
Twilight: 17
Rainbow Dash/Fluttershy: 16
Pinkie Pie: 14 (or 13 at the youngest)
Here is evidence to back this up Rarity owns her own company and lives by herself, Applejack's company is Family owned, Twilight is a student of Princess Celestia, Rainbow Dash and Fluttershy both went to flight school together, and Pinkie Pie lives with the Cakes is immature and hyperactive always making jokes and laughing. That is how I ranged their ages