The twelve year rule went down to ten years under recent rule making changes.
http://www.lpgasmagazine.com/dot-cylinder-requalification-rule-to-impact-propane-marketers/Five year visual inspection is still in effect.
It reduces the period from 12 years to 10 years for cylinders requalified using volumetric expansion testing. However, the requalification period for DOT cylinders following a proof pressure test increases from seven years to 10 years
The intent was to steamline the rules and eliminate uneven intervals and dates, in addition to providing a little more "safety"
Charles