Will hot water deteriorate pvc over time and cause a oder?
I am guessing that is why they sell cpvc.

Piping is 40 to 50 years old

Most odors come from the minerals and what not in the water. Stagnant water will often create an odor, but if you have other high levels of minerals such as a high sulfur content it will smell even if you have been running it constantly. Cpvc is more chemical resistant and may be more resistant to highly acidic water therefore capable of lasting longer than pvc.

Best piping to use for water these days is PEX. It’s easy to install, it won’t deteriorate easily, and it will expand 3 times its size before splitting if frozen.

Hot water doesn't go in PVC. It will get all droopy and not hold its shape.

Saying that drains are pvc and hot water runs down them so I guess it depends on what you are talking about exactly.

Kitchen or bathroom? Possibly the vent isn't installed properly. Or could be gunk in the P trap