I think by the time you reach your 70s and 80s, it's late to change lifestyle factors, i.e. a bit late to stop smoking, take regular exercise, avoid heavy drinking, avoid drugs, and maintain healthy eating/weight. The damage will mostly have been done.
But, if otherwise fit and well going into old age, things like daily/regular walking, keeping the mind active (even if simple things like crosswords, jigsaws, etc), keeping up with current affairs, i.e. reading a decent newspaper and/or watching the news, going to physical shops/library regularly, just for social interaction, joining clubs/societies, generally keeping an interest in "real life".
Basically anything to stop the "stagnation" that can so easily happen when someone has little to do, and where one day of "snoozing" and doing very little, quickly turns into 2,3,4 days, then a week, then two weeks, etc.