I think it's natural to feel homesick, especially during this first week when you don't have lectures to think about. Best advice I can give it try to keep yourself busy, meet new people, attend whatever event's going on, and start preparing for next week. And don't forget that home is only a...
Yeah I don't feel "mature" either... I'll be turning 25 on Sunday :)
Unfortunately I'm not staying on campus... altho reading posts here makes me wish I was.
What's funny is that when I graduated from high school, ANU was my first preference. It took me 7 years, but I finally got in!