For me, minimum age for a motorhome was 10 to 11 years, because of a State Law that says ANY vehicle 11 years old or older can be PERMANENTLY registered. Yep, I never have to be concerned about the registration on my motorhome expiring as long as we own it!
For a trailer, though, it is different. ALL trailers, new or used, get PERMANENT registration. Therefore, if I were to buy a TT or fifth wheel, overall condition is the primary consideration. Age is a non-issue, IMO. I would not buy new due to the price and depreciation.