Lauren Faust has already said the mane six are between the ages of 12 and 17.

It don't matter to me as long as they are 'legal'.

Ah… I'm a terrible person.

Though seriously, I think of them as this:
Twilight: 22
Rainbow: 19
Pinkie: 20
Fluttershy: 21
Applejack: 24
Rarity: 23

I count Rainbow as the youngest because of her personality, being the most competitive and brash.
Twilight because I count her as in the middle of college life.
Pinkie because… Pinkie.
Fluttershy from that remark.
Applejack and Rarity are counted as the oldest since they seem the most mature of the group, and as a southerner, I know how being stubborn is part of anyone's attitude here, regardless of age, so AJ being stubborn does not count as evidence.