I'm not all that old (early 50s) and in good health, as is my husband, but I'm only too aware that nothing can be taken for granted. At the moment, the most important things I can do are to make sure "my business is in order" - making a will, making sure my husband or other executors are clear about what financial and professional dealings I have and where to find them, and, at some stage, setting up a Lasting Power of Attorney and making decisions about end of life care, which, I'm sure, will change over time, as has happened with my parents.
My parents are both still alive, in their late 70s, and active and independent; life expectancy calculators suggest they could be around for another 15 years. They're orderly people and have taken steps to ensure that when the time comes I won't have a load of historical/sentimental tchotchkes to rehome and that if one of them needs care at home their house could be adapted very quickly. They considered downsizing but feel this would be more upheaval than it's worth, and it's not as if their current house is massive. They've discussed their plans for their estate with me (their assets are simple but our family structure, though not sprawling, is a little complicated) and I'm trying to persuade them to give me, as the person most likely to take responsibility for their care in old age, Lasting Power of Attorney. In the past they've always been like, "Oh, no, you're not getting at our money while we're still alive," but I'll keep mentioning the subject from time to time.
As far as assisted dying goes, my mother had a brush with death a few years ago as the result of a complication of a routine operation, and at the time she told the doctors in intensive care that she wanted to be allowed to die, and the consultant told us that they were obliged to respect her wishes. With the full support of the family, my father said that this should be overruled, and she made a determined and spectacular recovery. And as time has passed, my father's previous insistence that he should be euthanised if he becomes physically dependent has melted away.