My parents have already gotten old.
A lot depends on the health of the person(s) involved. My dad is 80 but in excellent health for his age and still very able to take care of himself. Actually, until February he wasn’t just taking care of himself but also mom, who despite being younger was in very bad shape with multiple problems. He was able to care for her at home until the end, with me and one of my two sisters providing some major support the final two months of her life. As I was unemployed, I was able to essentially move in with them for a couple months. If I had been employed I would have opted for Family and Medical Leave with much the same result.
Now, as it happens, although dad is still in very good health he chose of his own free will to move in with my sister in Buffalo and her family. Her house has a “mother-in-law” apartment, which gives dad a place of his own but he’s near family and if/when he needs help it will be there for him.
That’s how my family has dealt with this issue.
Now, what about YOUR family?
Have you even asked your parents what they prefer? That’s a first step.
As for the rest - well, assisted living CAN be good, but I had an aunt who moved with her husband into one such arrangement and it became a total nightmare and it wound up not only seriously endangering my uncle’s health but also cost them tens of thousands of dollars. On the flip side, my grandmother was in an assisted living facility (before that name became trendy, but that’s essentially what it was) and she was very happy with it, and was able to live in her own place until the last few weeks of her life. So, please, RESEARCH any assisted living arrangement you are considering. Some are good, some are bad.
Having mom or dad move in with you can also be very good or very bad - it depends more on the personalities than anything else. “Mother-in-law” apartments can go a long way to ameliorating the potential problems.
Having one party move closer to the other is another solution - you’re not in each other’s living space, but close enough to help out. It can also be very disruptive, stressful, and in many cases costly or impractical or imposing an awful lot on other family members, both adult and children.
There’s no one perfect solution - you have to do your best to find what fits your situation.