I looked at the Australian Standards for Fire fighting helmets ASNZ 4067: 2004 and could not find much on the life span of ten years.
As for the batch testing that is 9 helmets out of every manufactured batch which are then subjected to tests involving- Electrical insulation, Flame propagation, Convective heat exposure, Face shield tests, Impact energy attenuation, Penetration resistance, Retention system tests (chin strap) and Ear and Neck protection testing.
The only other thing stated from some manufacturers is that if the helmet is damaged in some way it should be returned for testing and if used in Compartment Fire Training it should be covered in an aluminiumised shield.
If the CFS is telling me ten years and my work is telling me the same that is good enough for me and that is where the so called "fact" came from as it is what I've been told to do from the Services I respond for. Maybe CFS got the ten years from a Manufacturers standard or a standard put in place by the CFS. I'm not really getting bent around the axles over it!
I'm waiting for a rep to get back to me from a company I deal with, which supplies my work helmets. Maybe he will have the right amount of years a helmet can live for.
As for the date stamp 24P I got told ten years from that date mate.