There's know set time when a trailer is unusable.If it is taken care of it will last forever just like a car or truck...We have an abundance of older trailers around here that have been used on nothing but gravel/wash-boardie roads for camping/hunting or fishing from the 60's on up..I own a 1972 Security 24' we use for hunting mostly along with my 2010 Keystone.
A quick look at Craigs list showed several in the first two pages out of Boise...
Example:
1969 Kit Companion - 17' - new carpet, tires, rims, propane tank. Everything works, refrigerator, stove, heater. Clean and 'road ready' Must See