Another fish question...My platy is dying and I want to assume it's old age?

Hi, another fish question. So lately in the past couple of weeks my two dalmation platies have been getting less active. The water parameters are fine, and the other fish are fine(for those who have seen my last question, my red platy is now doing fine it's peel is starting to heal up). It's just with my two platies it seems like their times are almost up, especially with the male he's been lying down on the ground. Whenever I walk by to check up on him he tries to swim up(maybe he wants me to think he's fine or something). But I got them like 5 months ago? And I'm pretty sure that they were old already since they were both and inch and a half when I got them. So, I just want to make sure that these fish are just being inactive because of old age? Or it's not some disease they are prone to get and I'm unaware of it? And if they both seem to get to the point where they can't take care of themselves anymore should I do something? Or just leave them alone until they die? Cause I don't want to do something inhumane, Thank you!

Note: I forgot to explain about the female platy, she isn't lying down on the ground like the male, but she has been staying in the back of the tank at the top, being inactive. They both are alive, but I think they are just getting old.