Which do you think wears out a car more - how old it is? or how far it’s gone?
Both. Some things wear with age like tires, seats, rubber, etc, other thing wear with use, like brakes, engines, transmissions, etc. body panel age with exposure to the elements.
Additionally, the lack of adequate maintenance will take a severe toll on an vehicle–whether it is a supposedly “reliable” marque… or not.
Also depends on the kind of miles. 50,000 miles of jackrabbit starts and challenging your buddies at the local race track is a lot harder on the car than 100,000 miles of long distance highway cruising.
All of the above, age, mileage, maintenance, type of use plus the environment it’s used and stored in.
A car in a moderate, humid, salty environment can expect more than average rust damage while the same car in a hot, sunny and dry environment will suffer more than average UV and baking damage, especially to plastic components.
The point is that used car hype like a “California Car” or “Ultra Low Mileage” can be completely meaningless.
Looking strictly at “wear”, it’s all about how far it’s gone and how it was treated while it was going.
If you bought a new car and parked it on the street un-driven for 20 years it would have a number of problems, but wear would not be one of them.
When the car passes 20 years of age it seems that the age related issues tend to outweigh the milage. For instance a 25 year old car with 100,000 or 200,000 miles will both be cheap.
With that age you get issues with parts availability, electrical issues start showing up, sealed lubricated bearings in belt driven accessories lose lubrication. Plastic parts get brittle. More modern shorter life rubber starts cracking. Paint starts having issues related to years of sun exposure. Plastic parts around the windows start cracking. The AC starts getting more refigerant leaks. Hoses get very stiff and difficult to remove and work with. The platic on electrical connectors break when trying to remove them. The dash starts getting cracks in it. The headliner falls down. The seat belt stops spooling back in on its own. The power locks start getting stuck. Discoloration of plastic body parts and head light covers can happen. The parking brake cable starts sticking.