Yeah. Like the majority of the posts above mine, I thought the mane 6 are teenagers. I'm guessing from 17-20ish. But.. the younger sisters (and poor scootaloo) are in elementary-type school. So that puts them at roughly 7-10ish. It is in fact strange that (most of) the mane six live on their own. But that's using our human logic in ponyville...
I dunno, man. It just feels right saying the mane 6 are teens.