Age is irrelevant to my way of thinking. Physical limitations are not. Nor are they always tied to age.
So the fact that you are 90-something has little to do with whether you should or shouldn't be boating. If you really want to do it, you should do it.
But it's important to approach boating sensibly, whenther one is 19 or 90. The previous comments about ladders, flying bridges, interior steps, wide decks, manhandling a boat at a dock are all things to consider. They are not reason not to get a boat, but they are guidelines for what kind of boat to get.
Only you can determine what you are capable of doing effectively and safely. If hauling a 30,000 pound cruiser up to a dock with a line is more than you think you want to have to tackle, no problem: don't buy a 30,000 pound cruiser.
If crawling around in a cramped engine room is not something you think you should be trying to do, either have the ability to pay somebody else to crawl around in a cramped engine room, or get a boat that doesn't require that. Maybe something powered by outboards or a boat with easy access to just one engine.
As to recommended makes and models, there are a zillion of them out there. Your sailing experience probably acquainted you with some of the more popular makes and models of powerboats.
A really nice boat layout can be found in the lobsterboat-style boats. Most commercial lobsterboats don't have flying bridges, and some of the recreational versions don't either. The cockpit and main cabin including the helm station is all on one level. There will be a few steps down to the berth and head compartments up forward, bu that's it.
The couple we boat with a lot have a 36' custom lobsterboat built on a commercial hull from Maine. Our boat, which is the same length, weighs 30,000 pounds. Their boat, I believe the owner told me, weighs 16,000 pounds. A lot easier to pull around with lines at the dock.
Emjoy the boat search. When you find the right boat, you'll know it.
First photo-- Commercial lobsterboat, Prince Edward Island, Canada
Second photo--- Our friends' custom lobsterboat.