I have a 7-oz that came new with a KP-3 rifle, dated 6-88, which would be just over 15 years, and I have a 20-oz around here somewhere dated '86 some time. It has an "adapter"- factory installed- to use a now-standard-thread CO2 pinvalve with the tank neck's larger threads.
I also had a 20-pound tank from the gas supplier (we just exchange 'em) that had first been hydrotested in 1967. I have a photo somewhere- it shows the series of test dates stamped all over: '67, '70, '73, '76, etcetra, on up to the year before I was using it (last summer sometime.)
I also have a 15-pound CO2 fire extinguisher that has been hydroed exactly twice: once in 1941, and again in 1944. It has an engraved brass band, badly tarnished, that says what ship it was on in the US Navy.
Doc.