Mine is an '87. It still works for what we use it for (mostly forest land). But it's starting to show it's age in not so great ways. I posted pics of big cracks above the door a while back.
So the answer is that they can last a long time. I think a 1987 trailer that still works good is not far fetched if taken care of. But I believe it's at a point where it will be costing me more than it's worth.
Here's my take on it. I buy trucks and cars with the intent on using them until I just can't anymore. I don't upgrade at any particular year. My F250 is a '92 and will likely last 10 more years before it doesn't make sense. I look at trailers the same way. But there's a problem with that.
Trailers don't seem to be built as well. Some better than others. And their value doesn't hold at all. Even worse, is some "RV resorts" have age restrictions. And that age tends to be right around 10 years. Some people have never encountered this, but I have. I know others who have. I've been turned down just based on age. I've been turned down based on color and graphics scheme. No kidding. In the 80s, they were all tan with red and brown stripes. Mine is, for the most part, in great condition. Looks like it did in '87. Except the graphics are faded. That right there got me denied at two spots. Both in Nevada close to Carson City. Now with those two cracks being epoxied, it's gonna be worse.
For the record, I called about 11 campgrounds in both Oregon and Washington for an upcoming vacation. None of them had age restrictions. Most had never heard of it. So that was nice.
With age restrictions and it starting to fall apart, we're upgrading next year.
I'll also agree that 10 years seems reasonable. But I bet after 5 years, you'll not get much money for it. I'd try to private sell it first.