There is one thing that I know of in the show that helps us pin down the Mane Six's ages, and it came in a small exchange between Fluttershy and Pinkie Pie. Pinkie Pie: 'Don't worry Fluttershy, aunt Pinkie is here!' (In 's not "s because I forgot exact quote)
Fluttershy: "I'm a year older than you..." This is straight from the horse's mouth (pun intended), so we've got to factor that into our figuring. That's why I'm pretty much sticking with my previous guess, but I'm increasing ages a touch and the order is also going to be slightly changed (As of now I'm watching S4 so I feel like I have a better handle on this; took a break to play games & stuff). Previous: ================ Rarity: 18 Twilight: ~16-17 AJ at ~15 Fluttershy and Rainbow: 13 (After all they attended Cloudsdale together)
Pinkie: 12. New: (Little more vague; I'm already no good w/ ages and the newer episodes made me doubt myself) ================
Rarity: 17-19 AJ: ~15-18 Twilight: ~14-17 Fluttershy and Rainbow: ~13-16
Pinkie: ~12-15
So yeah. If you think my previous idea was better, please explain why as I desperately want to pin their ages down more accurately. Also I can't help but think that Princess Celestia's ~20,000 - 24,000 years old and her sister Princess Luna is 17,000 - 21,000 years old, and that they age at the rate of 1,000 years = 1 year (So basically 20-24 and 17-21 respectively). Again, I'd love feedback here