I also see them as ... well about 18-23. They're old enough to be out of school and have jobs, but young enough that they're still learning to open up and socialise in the real world so it seems about right.

Don't like to see them as less than 18 because rule 34, heh. But honestly don't see them as less than that in the first place.

It's all relative though as they're, well, ponies.