I would guess the ponies to be older than what Faust says they are. I would range their ages at 22 - 20.
From oldest to youngest:
Fluttershy - I put Fluttershy as the oldest, 22, mainly because she was just so much bigger as a filly in "The Cutiemark Chronicles". Her legs were super long and she was just taller. Also, she claims she is a "year older" than Pinkie Pie.
Rarity - Rarity is the second oldest, maybe a couple of months younger than Fluttershy. She's just very mature & seems very well educated. Which of course, could come from a college education. Also, she's got her Boutique, and, I don't see anypony under 21 (not to say they can't) doing what Rarity does.
Applejack - I would say Applejack is a young 22 OR an old 21. I'm saying this based upon her sibling's ages. I would put Applebloom in the range of 9 -12 and Big Macintosh in the range of 25 - 30. Applebloom is quite a few years younger than AJ & Big Mac is quite a few years older than AJ - old enough to play an authoritative role (she says to her sister that she'll "tell Big Macintosh" on her). So based upon Applebloom & Big Mac, I get 21.
Twilight Sparkle - I put Twi at 21. I'm sort of just basing this off the age I believe Shining Armor to be - 25 -31. That puts them, at max, ten years apart. Which, if you saw in the scenes of the song "Big Brother Best Friend Forever" makes some sense.
Pinkie Pie - Pinkie's 21. Loves to party, pull pranks, etc. She's just young and full of life.
Rainbow Dash - Dash is the baby - only 20, but an older 20. I say this mainly because of her maturity level.