Hipman, you're bummin' me with your age-related malaise. I'm 50 and just started riding in August. All I can say is that most weekends I take a 40-50 mile ride which I could not have imagined doing when I was in my 20's. I was mostly chasing women and they usually didn't run THAT far! I've been riding around 100 miles a week and it feels great. I've been a whitewater kayaker for a lot of years but never developed any real endurance base, so riding a bike has been a fun revelation for me. Now I just need to find somebody to ride with that's older and slower than me so I can wear 'em out! If you want to feel young, go to a 30 year high school reunion (it doesn't have to be yours) and see how many people look better than you do.
PS: If the answer is "Most of them", get back on your bike or try another school.