Parts that don't work
I think the hardest thing about getting older is that your parts start breaking down. There are distinct advantages to age. Time and experience are tremendously valuable and very few people would like to have an 18 year old's maturity. But a lot of folks want an 18 year old's body, firm of skin, easily recovering from injury, and slim of figure. That seems to be the ideal combination, an 80 year old soul in an 18 year old body. I'm working on the soul thing, but sadly I think the body is beyond hope.