I once bought a 13 year old TT. The previous owner and I both did engineering field work and basically full timed in it while on field assignments. Assignments could be from a few weeks to several months, so it didn't get a lot of mileage. The original instant gas water heater had been replaced with an electric domestic unit. It would be considered pretty crude by modern standards, but there were no inverter, converter, circuit boards or electronic gadgets to go bad. It was 22 years old when I had to replace the electric heating element in the refrigerator. It was 26 years old when I sold it, still working.
We currently have a 1988 Terry 5r. Other than the tank gauges being of questionable accuracy everything is original and still working. The previous owner full timed in it for about 6 years and maintained it well, we have taken it to Alaska and back.
Condition and maintenance are more important than age.