Early 90s, NT 3.1 was released in 93. By release date, nearly 31 years old. First version that a significant number of people actually used was 4.0, 96, and the first consumers used was 2000. Development started in 89, as OS/2 3.0, then still a joint project with IBM.

For comparison, the first public Linux version dates to 91, 1.0 to 94, though Unix of course goes back to 69. Practically the only other thing still surviving from that era are IBM mainframes. In CS terms both are prehistorical, you can tell by how papers from back then aren't typeset in TeX, worse, are usually scans. Yet they somehow had it all pretty much already figured out and we're now often re-discovering insights that they simply didn't have the hardware to implement back then. Not impossible to implement back then but here's a fun one. And a modern walk-through through the thing.