On my mom's side my great grandfather was always a fixture at my grandparents. In his chair, watching TV with headphones and a curly cord snaking across the room to it. When we came back to the States in '85 he moved in with us, and lived his last three years until he was 98 with us.
He was 'healthy', but aged and slow. Walked out each morning before breakfast to pick up the paper to read after breakfast. One day he told my mom he didn't feel well. Ended up checked into the hospital with his heart weakening. Lasted less than three days, enough time for my grandparents to fly down and see him one last time.
The grandfather on mom's side was diagnosed with Parkinson's right after he retired, made it to his early 70s. His wife lived to be 91. She lived with my parents the last three years of her life. Short term memory completely went, but she always recognized all of us and was usually happy. One thing that did stick in her memory was that I was going to be getting married. I swear she hung on for that, because she always asked my mom about it, and after we married October 2010 she passed away the Wednesday before Thanksgiving.
Grandma on the other side passed away at 87, in a weird coincidence on the Wednesday before Thanksgiving the next year. She had been in a memory unit with Alzheimer's for several year, and didn't recognize her own kids, much less grandkids. My grandfather passed away at 88 from a brain aneurysm 28 days after she passed, in the middle of eating his dinner. Uncle found him the next morning.
If I ended up losing my memory the way my grandmothers did, I wouldn't want to be around. I couldn't actually do anything I enjoy.
If I was as 'with it' as my great grandfather was, I'd be ok with living to 98.