Little update on the human lifespan section... Not that it's fully 100% GW fluff, but in Dark Heresy (which is GW approved at least), their is a Daemonhunter Inquistor nearing the end of a phenominally long life, and he's in his very late 200's approaching 300.
SO the default human bodies (non-chaos, non-marine, etc, just good medical care and naturally healthy), absolute, maximum extreme human life would appear to be the low 300's from that source. Which seems to jive with most other sources, and strongly fits in with the fact it would mean the AVERAGE Marine outlives by about 25-65% the EXTREMELY LONGEST lived human, and some rare chapters outlive it by 5x that amount, and some rare individuals of those rare chapters can hit 8x-9x.
Although again, the overwhelming majority of Marines die long before they'd ever die of old age... and I'd have to imagine if a Marine WAS nearing old age, he'd probably try and take bigger and bigger risk. I can't imagine culturally their is much pride in being a Marine who dies of old age. It just means you weren't fighting enough. What, do you not love the Emporer enough to die for him? What's wrong with you slacker!!! ;Y