Based on what I saw from a post on Facebook, Rainbow is 18. Still don't know how old AJ is, but I also learned Celestia is 1000 years old-and she still looks beautiful.

I'd say Rainbow Dash is at least a year older then Applejack.

I believe Applejack is 18, and Rainbow Dash is 16.

There are 21 candles on Rainbow Dash's birthday cake in Pinkie's Pride.
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Applejack, but to be honest, I don't really think about it often. XP

The real ages are unknown, but I get AJ is a bit older.

Applejack is a tad more mature.

As izfankirby has said there are 21 candles on Rainbow Dashes cake and if each season is equivalent to one year then Rainbow Dash was around 17 when the show started. There hasn't been an episode (I think) with Applejack's birthday, but she's always seems older to me. I think Applejack and Rarity are the oldest.