In reply to the OP's question: "Let's pretend that a miracle drug unlocked the cell reproduction cycle allowing people to maintain a 20 year old like body for 100 years, with a total lifespan of 400 years. How do you think these people would act on their 100th birthday- youthful and energetic or old, want to stay at home type people?"
Youth, in this case, would definitely be an enabler. When I was going through high school and early college, I observed a lot of people doing stuff I would never do - partying, going out to theme parks, drinking till they couldn't stand up anymore (every night), etc... A lot of these people are married now, not as crazy, and seem to be spending more time working.
I like to think that I'm getting wiser with age. However, it seems, the older I get, the more I know I don't know (affecting my confidence). My great grandpa died when he was 101.
https://goo.gl/photos/2SEBQZA2tVC8wVa59 for picture of him and his wife. Every day he would go out and tend his garden even when he was 100. He was pretty quick mentally even near the end of his life - recalled using a horse to get to school, etc... He didn't last long after his wife died.
Several of my grandmas say they feel the same as they did when they were young.
I'm 31. When I hit 30 I felt worse than I ever have in my entire life because, last year, due to work stress and overeating I got to the heaviest I've ever been (235 pounds). I couldn't run a quarter mile without stopping being out of breath. After 6 months of exercise, biking, and a lot of work - I've lost 15 pounds, I can run 5 miles without stopping and I can bike 40+ miles... I feel much better. But I still don't feel as young as I did when I was in my 20s (180 pounds). I imagine losing more weight may help with that but I'm in the best shape of my life at the moment.
So if you asked me, if I hit 100 with a 20 year old body - would it change anything? Only if I had more financial security. My first thought would be how awesome it would be to have so many more years to read books, learn new things, and spend time with people I love (knowing I could live to 400).
This is from the perspective of someone who hates social gatherings and has never been to a concert - would much rather go to a library, a museum, or watch a documentary on how something is made. Being the type of person that prefers to sit at home and read a book makes me very different from a lot of people.