I'm going into my Senior year after I get back, so I've waited until kind of late in the game. I don't know if it matters all that much when you go, to be honest. If you wanna have a free reign at PI and take advantage of all the clubs, then waiting until you're 21 might be good, but if you're only 18 now, that's a while to wait. You can't think about it too much, really--you just have to do it.
I interviewed at my school ("audition" might be too strong a word). I don't know if it's the same everywhere, but with the guy I talked to, he seemed intent on asking people questions relating to their majors, asking them why they thought they'd be a good addition to the Disney cast, and tesing their enthusuiasm (and putting them on the spot a little). He asked me why I (as a Journalism major) would be better-suited than a Hospitality or Business major, and then asked me a question about what I'd do in a situation with a kid separated from its parents. I think I sold him by telling him I was just interested in learning more about people and just interacting with them on the first question, and then just gave a common-sense anwer to the second one. ("Keep kid calm...contact security.") I know he asked a friend of mine who's a Marketing major to "sell" an inkpen to him, so just be prepared for stuff like that, and make sure to show your enthusiasm. I think a BIG thing with them is weeding out the folks who are for real from the ones who wondered in for the donuts and free stickers.
As for housing...yeah, expect a lot of flatmates, but I think it's worth it. Having lived in a dorm for the past year, a real apartment is awesome to me...and you can limit your roomies somewhere between 3-5, I think, so yo don't have to be in a house with 6 people, if you're willing to pay more rent.
OK, I've blabbed on long enough; anything else you wanna know, ask here or just PM me sometime.