20 year old hypalon inflatable boat still usable?

Hypalon is super tough and known to last for a really long time. Definitely do an air test over night to check for leaks. Stick close to shore initially... you will have a pretty good idea right off the bat if there are any issues. If you have no leaks the only other issue could be the material being worn thin from exposure to the elements. I would recommend getting a UV protectant spray which you can use to preserve the material from here on. Good luck!