So basically, if they were human they'd all be in the 18-25 age category: young adults, Twilight being in college, Rarity and Fluttershy being graduate level in maturity and independence, and the others scattered around not far behind them. However, seeing as how they're ponies and age differently than humans, it is reasonable to cut that by several years because ponies don't generally live as long as humans do. And as was mentioned on the previous page, Lauren wanted to make them more relatable to kids.
But as far as we're concerned, relating them to ourselves, the 18-25 thing fits best.