Some constraints:
They live on their own.
Fluttershy is one year older than Pinkie Pie.
Princess Luna were banished to the moon for 1000 years.
Princess Celestia is older than Princess Luna.
Princess Celestia is around 10,000 years old in the premiere
Despite the fact that Granny Smith looks pretty old, Princess Luna and Celestia don't.
Spike is younger than Twilight Sparkle, (I know that Spike is not a pony).
We know that Twilight Sparkle has coursed magic kindergarten, so we can assume that she is at least more than 6 years old.
Since no pony is studying in a college, we can assume that they're at least 17 or older
Using flashbacks and sizes for reference:
Fluttershy when she got her cutie mark was biggest in size.
Rarity was allowed to use sharp things.
Applejack traveled without A guardian.
Therefore according to me:
Applejack: 20
Rarity: 20
Twilight: 19
Fluttershy: 19
Rainbow Dash: 18
Pinkie Pie: 18.