[phil_h]

What ages rubber prematurely is heat cycling or oxidisation from either ultraviolet or ozone.
Therefore even if you keep a tyre in the dark it will harden prematurely if its not kept cool.
It will prob still look ok, but wont grip.
It is also sensible to keep tyres with steel carcasses away from moisture, and hence you should keep them above the dewpoint to prevent condensation forming on the inside.