Originally Posted by LHaven
I forgot how long cylinders go between certifications, but I'm assuming that all the left-handed thread ones are now all older than that, and no filling station will fill a tank without current certification, so I have no idea who's buying these propane adapters.
As far as I know, at least here (every state can change the federal rules to fit their "more stringent requirements)... New tanks are good for 12 years and can be "visually inspected and recertified for 5 years" or they can be "pressure tested and recertified for 12 years". There is no limit on maximum times a tank can be recertified, and I've seen some "high dollar aluminum tanks" on vintage Airstreams that are 40+ years old and still in use. The OPD valve is the "stickler" and a up to date valve can cost as much as a new 20 or 30 pound tank, so many people who want to recertify their tanks, when they find it costs as much as a new tank, will just buy new and discard the old ones.
But, I don't think there's any "age limit where the tanks can no longer be recertified" as long as they still meet the inspection/recertification guidelines.