I traveled Europe for a year when I got out of school. One of the guys I hung out with most was a 40+ year old from New Zealand. Great guy, we got along well. This was at a party hostel. I think it's a matter of your personality and comfort level. Worst case scenario, you sleep, get up, and go spend the day doing the things that interest you without ever hanging out at the hostel. I don't think there's anything weird about 30, 40, 50 or older staying in a hostel.