For me it's drinking. I had great times drinking liters of beer in my young and going to live forever days, especially at Oktoberfest or some bar in Heidelberg, drinking the "boot" or whatever it was called. Getting blasted on the Heidelberg bridge on New Year's Eve with friends. Wine fests. Beer fests. It was all great.
Doing shots of Jagermeister, which tasted like Vick's Formula 44, and ouzo. I'm surprised I'm still alive.
And then, sometimes, getting up the next morning for work and feeling relatively fine.
Now? I like a beer or two, but any more than that and I feel lousy. I guess the brain and body just don't metabolize alcohol like in our younger days. I guess it's true that Father Time is undefeated and untied.