Make sure they keep on top of getting sight & hearing checked, and encourage getting things fixed - my MIL has quite considerable cognitive decline, and with a bunch of ableism and anxiety resisted things like cataract surgery and hearing aids which I suspect could have kept her more engaged with the world for longer - I don't know if it would have made a BIG difference but still worth doing.

Also, support them to move to more supported living arrangements (or living arrangements with step-up care available) before they need it. It would allow better decision-making, and allows more agency to the aging parent.