Rubber Aging - effect on bushes, hoses, mounts & vac lines ?

[BrazBenz]

A few days ago I observed some 12 year old spring pads of a BMW. Not rotten, broken or something like that. But they where harder than the normal, bound to brittle.
That is why I am raising a generic question:
Is there a consensus on replacement of this parts due to aging ? (eg. time interval)
I think of the heat exposure of the engine mounts, for instance. For 12 years... How are them really even if they LOOK perfect?
About the suspension bushes: what is the net effect of the potential hardenig versus the obvious objective of being flexible and providing a pleasant ride and vibration dampening ?
Comments are mostly welcome.
I would particularly appreciate guidance on the different parts categories. (some get a lot of heat, others weight + heat, others torsion + weight...)
Thanks, Jorge