I'm 35 and don't feel old at all. I guess because I am not. One thing I have learned over the years working in an emergency room, is some people live over 100. It's all about how you take care of yourself.
I do however notice hangovers to get worse as the years go by. I use to drink a 5th of hard liquor, or when younger my friends and myself would do dumb shit, like split a bottle of 151 between 2 people, drink it straight out of the bottle, and chase it with a carton of orange juice. Then there were the bonging of 151 mixed drinks, normally followed by gravity bongs in the bathtub, followed by WCW NWO Revenge on N64, and watching WCW back when it was cool.
Now I buy a 6 pack of Newcastle, drink 4 and want to go to sleep. More than 6 and I get a hangover for two days. Not sure if I am really getting old, or my tolerance went down from dropping drinking to only a few times a year.