I won't speak in pony years since I don't know how that works. So I'll pretend like they age like humans instead. I have to go with Rarity being the eldest, just from what she has going for herself having her own business and what not. Not to mention she's the only one to show interest in the opposite sex. I'd say she's the equivalent of a 21 year old woman. I believe Fluttershy and Applejack are probably the same age and second oldest. Applejack's maturity and work ethic isn't something common in younger folk in this case I'm assuming ponies too. I'll say they're about 19 or 20. Twilight, Pinky, and Dash are probably really close in age, so I put those three at 18.