Except for axle bearings and brakes, I don't really think towable RVs "wear out." So age doesn't matter that much, condition matters. That will depend more on how well it was taken care of, and how stored when not in use.
It is not unusual to find 10 year old TTs in almost new condition, seldom used and stored under shelter. 20 to 60 years old, the really great ones are rare finds, but they exist (we have at least three in that age range, in our RV club).
It is quite possible, however, for a TT to be coming apart at 3 to 5 years, if it got wet inside, particularly if it stayed wet inside.
However, there may not be much difference in price, beyond 10 years, if a TT is in really great condition. The market recognizes a floor on depreciation, a residual value that comes of something being of useful service. And when RVs get really old, they can become expensive classics, even collectible.
Tom Test
Itasca Spirit 29B