What legislation or regulation mentions this age limit? Regulators are not even date marked. When they fail, they fail safe. The regulator is included in the pressure drop test as part of the annual service.
I am not aware of any legislation or regulation concerning this but various interested parties in the industry put a maximum of 10 year life on LPG regulators of the type we use.
Just a couple of near related examples LINK 1 and LINK 2 both touch on the subject.
There are differences the more you look into this between, "from date of manufacture" and "date first put to use".
As swelling of the control seal due to exposure to LPG is sometimes claimed as the underlying reason to change, then date of manufacture has little technical relevance, but is likely to be the only tangible one, even if available.
The date of the body casting I have found as a circle character, cast in the body, but that is no assurance when the casting was made into a regulator.
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By comparison, but with no direct connection, on our natural gas domestic utility that we have had for 42 years, whilst the meter has been change the regulator has only once been changed.
There these have way exceeded ten years use, with exposure to that gas 100% of every minute of those 42 years.