My thoughts on how old the mane six are:
First off, Rarity has to be the oldest, at least twenty years of age.
Second, Fluttershy claims she is a year older than pinkie pie, so I expect it to be 17 and 16 respectively.
third, Rainbow dash should be at a close enough age to Fluttershy to have been her friend in flight camp.So, RDs age should be between 16 and 17.
with applejack, she could be at least 18, considering she works at sweet apple acres and handles the cider they produce.
For Twilight, Im not so sure. I would expect her age to be close enough to Rarity to understand her well and young enough to tolerate Pinkie pie.So I would say about 19 years of age.