As Peach indicates, somewhere around the "young adult" age, so anywhere from 18-28ish at the time of episode 1. I believe Rarity is the oldest as she has a well established business by the time the first episode starts. We also know Fluttershy is 1 year older than Pinkie Pie (Episode 5). Twilight is likely somewhere in the middle of the pack. I would guess Rainbow Dash is the youngest, though somewhere around Pinkie's age, since both Dash and Fluttershy attended flight camp together. These are all guesses, of course. Which was a delicious moment, pun completely intended.