To me what matters most is how they were taken care of, more so than the hours. Also, one of the worst things for any type of equipment is for it to sit around unused. This causes seals to dry up and crack, corrosion on internal components, fuel systems fouling up, hoses, belts and tires to dry rot, etc (this also partially has to do with age.) I personally would rather take a higher hour tractor that gets used often but is well taken care of than a 20yr old tractor with 500hrs on it.
We have an IH 454 diesel that has 7000+hrs on it. We got it with around 3000hrs in the early '80s and it had been run into the ground by it's previous owner. Installed a new set of brakes and clutch and it has been doing well ever since.
We also have a 2550 Deere that is about 25yrs old but only has about 2500hrs on it. I would consider it a pretty low hour tractor, but we have had to replace a few seals, hoses, etc due to it's age, not due to the number of hours.
At the top end of the scale, my uncle has a couple Allis-Chalmers with between 20,000-30,000hrs on them, a D-17 series IV and a 170, both gas powered. Ofcourse they have had a few overhauls throughout the years, new brakes, clutches, etc, but if you are willing to maintain a tractor it will last virtually forever.