[squarefoot]

My IBM model M I got in the mid-late 90s has a "Lotus 1-2-3 Release 2.2" sticker in the area over the function keys. That should make it about 28 years old and not a single key ever failed. Missing Windows keys are a nice feature also :)

[etcet]

It should have the manufacture date on the bottom. Mine's got a month until its 29th birthday.

[squarefoot]

The sticker underneath mine is damaged but readable: made in UK and the date is "1990-09-05", then other numbers one of which a lot bigger which could be a serial number (1391404). I have another model M somewhere, probably a bit younger than this one; they were throwing it away from a govt server room I worked in many moons ago because it didn't fit in a rack drawer. The crap they put there probably broke in less than 2 years.