From Lauren Faust's Deviant Art Page
Q: How old are the ponies?
A: The ponies' ages were left intentionally undefined. I like to think of them as having maturity levels anywhere between 12 and 18, (but remember that maturity is different than age. You can be a 10-year-old with the maturity of a 15-year-old, and you can be a 35-year-old with the maturity of a 15-year-old.) For story telling purposes, they needed to be able to live independent of parents, but innocent enough for the type of lessons they learn at the end of the episodes. Since horses are full grown around 5-ish, I guess I sort of justified it in my head that they could be childlike young adults. I know this answer is frustrating for some people, but it happens often in cartoons. How old is Bugs Bunny? Mickey Mouse? Sponge Bob? I don't know either.
So it's undefined, but it's strongly implied that they are 3-5 years old in pony years. I highly doubt this is their actual age, and I suspect MLP ponies age differently to RL ponies.
Edit: I've just checked wiki and the oldest age for a
pony horse is 30 years, though that is only the average. Some unusual horses live to be 40, with the oldest pony at 57. Let us assume that Granny smith is about 50. She's old but not so old as to be dying all too soon. If we make her at about 200 years old in MLP years (due to her being there at the start of ponyville which is centuries old), that makes an age contraction of about 1:4. Then checking back to Wiki, any horse pony older than 4 years is a Mare or a Stallion. I'd then place their ages between 16 and 20. We've got to bare in mind this though:
Regardless of a horse's actual birth date, for most competition purposes, an animal is considered a year older on January 1 of each year in the Northern Hemisphere and August 1 in the Southern Hemisphere. The exception is in endurance riding, where the minimum age to compete is based on the animal's actual calendar age.
I think we're safe in the assumption that we're in the northern hemisphere. From this all the ponies ages are 'the same', except Applejack who is probably considered a year older due to using her actual date of birth, I'd make her ~5 but only just and living with such young friends makes for her not acting her age. I'd probably sum all of this up to say they're all about 17-18 in MLP years (4-5 in RL pony years). In the episode "Griffon the Brush-Off" Fluttershy retorts to a line said by Pinkie Pie by saying "I'm a year older than you", which could either be interpreted in pony years or our years, giving the age of either 5 pony years or something like 19 or 20 of our years. Another thing is how Pinkie is actually far smarter than she initially seems, implying much more maturity (which again implies age) than at first glance.
That makes our best guess something like this (to the nearest year) in RL/MLP terms:
- Twilight Sparkle - 4/18 (fresh out of magic school)
- Pinky Pie - 4/18
- Rarity - 4/18
- Apple Jack - 5/20
- Rainbow Dash - 4/19 (rounding down to 4 due to the January birth thing)
- Fluttershy - 5/19 or 20 (based off of the questions premise)
- Princess Luna - Indeterminant, minimum 1000, although unclear
- Princess Celestia - Indeterminant, minimum 1000, although unclear
- Granny Smith - 50/200