QUOTE=fietsbob;13180756]Tube is not exposed to sunlight, being inside the tire..

they are Butyl rubber, petroleum based, .. latex in ultra light tubes may age faster.[/QUOTE]

It's not just sunlight, it's ozone levels too.
IMO it is also the chemical mix, wall thickness and cure procedure from the OEM that also determine longevity of the tube. You will also have to factor in inflation level (over/under), heat and impacts.
I recently resurected my brothers 1970's Columbia. The Taiwan mfg. gumwall tires were disintegrating(cracked, flaking), but the original Carlysle tubes still had about 25psi in them. That's after sitting for over 40 yrs.
I've got newer tube, tire combo's that require airing up every week. No visible leaks in the tubes, so IMO it's the schraeder valves that are allowing air loss. As my budget allows I am replacing tubes, where possible, to presta valve tubes. Will see how that works.