[Rubbond]

Actually there are several ageing processes involved, including physical
ageing, which applies to all materials - this is where molecules descend to
their lowest energy levels which is why you can't easily cut old glass.
With rubber the key issues are
1) heat ageing, which is basicly oxidation,
2) leaching or addition of plastisisers (softening oils) which won't
normally happen on the shelf, but will if you get engine oil on your
radiator hose, and
3) the effects of Ozone and UV light. As most rubber profiles are black, the
carbon black will absorb the UV energy and if stored indoors, the UV won't
have much effect.
With SBR profiles, used in the 1960's, the biggest culprit is heat ageing
and/or Ozone cracking. The double carbon=carbon bonds in the polymer chain
break open and the polymers strength has gone, like an old elastic band.
Polymers with a saturated backbone (i.e. c-c bonds instead of c=c) are far
less susceptible to heat ageing and UV exposure. These polymers are denoted
with an "M" as suffix, i.e. CM, EPDM, FKM, etc..
SBR, NR and BR, all used in tyres, are very susceptible to both heat ageing
and UV light, though being black, this helps the latter, but worsens the
former in the sense that they get hot exposed to sunshine. Ozone is what
causes the cracking. Unsaturated rubber molecules under tensile stresses of
about 10 - 15% (i.e.a tyre left flat for several months or the stretched
corners around your windscreen) are highly susceptible to Ozone cracking.
Saturated rubbers are not, hence the need for (in this case) EPDM based
compounds, which were first produced in about 1968. I have cracking however
on the rear (fixed) side windows of a 1998 Land Rover Discovery, where the
profiles were probably a bad fit when made, and are probably still SBR.
So there you go.
Probably best to buy a German car. Their specs are the best in the world for
all rubber parts, including washing machines.
Andrew.