how come my 21-year-old, well-used Stingray has almost no fret wear, and my 7-year old Fender Jazz 5-string has noticable wear?
It depends on many things. Nickel strings wear down frets less than stainless steel ones, and nickel frets are worn down easier more more easily compared to steel ones, those are just a couple of factors.
Do you have flats / rounds on different basses?
Well, I know that the owner of the Stingray used EB Hybrids on it exclusively for 20 years. The guy I bought the Jazz from said he only used GHS Boomers (nickel-plated steel) on it, and put on a nice fresh set of Elixers when I bought it from him. So both basses have only seen nickel-plated steel roundwounds.
Do you know what the frets are made of?
Your jazz's fret material is probably at a lower quality than your Stingray's.