Tennis, bike rides, video games, concerts, jogging, employment, chess, etc.
With the exception of professional sports or extreme activities (mountain climbing, etc.) there's very little that you can do in your 20's that you can't continue doing well in your 60's.
And when somebody dies from stroke/heart failure/cancer in their older 50s, 60s and early 70s, it strikes me as a much more tragic and untimely death nowadays. Sally Ride, the first female NASA astronaut, was still several decades away from the limits of a human lifespan when she passed away suddenly.
I'm 45, and my body started to rebel just under two years ago. I take longer to get moving in the morning and I don't have the physical endurance I used to. Of course being obese doesn't help, but I'm certainly not the spring chicken I used to be. Losing weight will help, but I'll never be a spry 20-year-old again (probably a good thing, as I was a complete unbearable know-it-all little cow at that age, and needed a good slap upside the head).
My parents were 41 when I was born, so when I was younger I didn't really think of people under the age of 40 as old (as opposed to just being grown adults) because of the large generation gap in my family. My mother died of cancer aged 73, and my father passed away two weeks ago from natural causes, aged 87. I agree that anyone dying before their 80th birthday seems "too young", because my parents as well as my aunts and uncles have always been so much older than me, and most have lived into their 80s.
As for young, anyone under 30 qualifies in my book. I still find it hard to believe that I work with people who were born after I graduated from high school, though.