Stay active. It really is “use it or lose it” with physical and mental abilities.
My aunt and uncle are both 90. My uncle has always used a treadmill or standing desk for his computer and pulls long hours in front of it still doing pro bono legal work even though he’s been “retired” for several decades. He walks to the grocery store and carries the groceries home. He walks barefoot around the block every morning and has a body weight fitness routine he does every other day. He’s doing just fine, his brain wheels turn a little slower but they work just as well as ever.
My aunt got very sedentary around age 75. Her mother developed dementia around that age and she just sort of settled in and waited for it to come. Maybe it is hereditary and there was no point in doing anything else, maybe not. She’s wheelchair bound now, just from lack of strength, not really any medical issue. She can take a few supported steps to transfer, but that’s it. Her short term memory is gone, I go have lunch with her twice a week and she knows who I am, but as I’m leaving she’ll say she’s sorry we couldn’t have had lunch while I was there and it’s a shame I can’t visit more often. It’s not really out of bounds for 90, but I’d rather take my uncles route than hers.