Slightly different take.
I am 68 and, I feel, reasonably healthy. I know people of around my age who are both more and less healthy. I can be active but am often sedentary (naturally lazy but make some effort)
I do part time relief work as a passenger assistant on the school bus runs that transport children with special educational needs and/or behavioural issues.
Most of the children we transport live in disadvantaged areas. The youngsters (13/14) have asked me a number of times how old I am. I have no issue with telling them. Today one said, 'How can you be 68 and look so young and fit?'. Kind of a compliment from a very challenging girl but as far as I can see it is down to a combination of lifestyle, opportunities and genetics (good skin is definitely genetic in my case).
The parents of these children do their best to give their kids a good life but most of them smoke, they haven't been given the right opportunity to learn to cook and live largely (if we are to believe the kids) snack/instant/fast foods.) (One girl goes to the shop before being collected and brings a 6 pack of crisps and snack biscuits to last her through the day)
In short, I believe genetics and luck make the biggest difference with lifestyle following on (but being at least partially determined by the first 2)
(Mum had 3 sisters, all different lifestyles and levels of activity with my mum being the most overweight and most sedentary. All 4 lived to their 90s (genetics) mum being the least fit)