Turn of the 90s easily, before tubes got a bit wider and thinner.
The early MaxOR frames had the stays detach in places (iirc the dropout?), MaxOR did make a great frame though and the 97 sorted it, but while you'll not really break it a and it rides beautifully, comfy yet rigid. The tubes are easier to dent than the early 90s with Prestige Concept tubing.
I think by 93 when weighs started to drop, they became not quite as tough.
Anything after 97, we don't consider of course...
I say all that, for me the early 90s frames tended to rust easier and get worming. (Just a MTB in general)
But that could be 6 or 7 years of extra time or just the way they were used and abused. (Probably paint technology)