The lifetime depends on who made it. Some of the original Jacobs machines are still in use today. Joeseph and Marcellus Jacobs were ranchers in Montana in the 1920’s and 30’s and developed a very reliable and cost-effective wind generator that powered many remote farms and ranches into the 1980’s, when the REA made great pains to make electrical service dirt-cheap for even some of the most remote customers.
Some of the older Jacobs machines are still sought by modern-day folks seeking to put up a home-scale wind generator for the quality of their construction.
Well made in the 20’s and 30’s, I would imagine that would be a simple and rugged design with GE generators mounted and ground level and easily serviceable.
The problem I see with the modern wind turbines is that the generators are mast mounted, gear driven and solid state regulated.
All of that means complexity that your average farmer (or anybody with out an EE degree) may have trouble maintaining on their own. The thing that someone buying these things needs to remember is that in 10 or 15 years none of the parts for these things will be available. If your voltage regulator fails or a gear in the transmission wears out they will most likely be obsolete and unavailable. Replacement parts available may be incompatible with the other parts of your machine.
And most likely those older windmill generators were most likely not mounted at the heights that the modern wind turbine are. How many of the old farmers would be climbing to 200 or more feet that the modern wind turbines are mounted today to change generator brushes.
My grandparents were farmers and would rarely ever take a machine out for repair. Almost everything on a modern wind turbine needs a professional technician to service it.