Ages:

Twirly Swirl - 20
Amethyst Star - 20
Spike* - 20
Flitter - 21
Rainbow Dash - 21
Twilight Sparkle - 22
Fleetfoot - 22
Pinkie and Marble Pie - 23
Cloudchaser - 23
Applejack - 23
Fluttershy - 24
Lotus and Aloe Blossom - 24
Shining Armor - 24
Rarity - 25
Maud Pie - 25
Saffron Masala - 26
Limestone Pie - 28

*Age accidentally affected by magic.