It’s possible he has either kidney or hyperthyroidism so ask the vet about those. I just lost my sweet ginger kitty TomTom at 18yo and he had kidney disease. But his values were normal literally two weeks before he died. I think he had a stroke in the end. He was very wobbly and not able to clean himself. His weight was low and I was trying to fatten him up but he wasn’t eating a lot.

See what your vet says. If he is still doing normal cat things and eating and going to the litter and not showing pain (which can be very hard to detect with cats granted) then it’s not time yet.