Lifespan of a Level D simulator is: as long as the maintenance costs are justified.
Maintenance costs of a simulator is according a typical bathtub curve, with the right side of the curve forever climbing up and up. Like with operating and maintaining an old-timer car, as long as spare parts can be purchased it is only a matter of justifying the costs.
New Full Flight Simulators are produced for new aircraft types. As long as an old aircraft type is still flying, there will be pilots that need to be trained on a Level D simulator constructed decades ago. There will be fewer and fewer of the old type simulators around, so training on an oldie may be very expensive indeed.
But spare part availability is the key. The real-time computers, I/O systems and analog electronics of the early 90s are really hard to find nowadays. But even then there is a solution: replace the old simulator sub-systems with modern ones. Host computer, visual, motion, control loading, audio, I/O, Instructor Operator System, weather radar...Not a cheap option, by the time all relevant systems are replaced the upgrade costs are higher than that of a newly constructed sim. But the costs can be spread over several years, with only the most pressing ones tackled year-by-year.
I know of several Level D sims constructed in the early 90s that are still in operation. Indeed the B737-200 in the museum mentioned by @ymb1, there is also one in Jakarta operated by Sriwijaya.
We're upgrading an 1993 Level D sim at the moment. Almost 30 years old, targeted future use of over two decades...