There are several age-related forms of deterioration in tires, but as far as protecting exposed (i.e. mounted) tires from suffering tread cracking and sidewall rubber "evaporation", the most important thing is to keep the bike indoors.
The less exposure to outside air, the longer the tires will remain in good shape. An indoor closet is almost as good as a plastic bag because still air in the closet (or any well-sealed room) has had a chance to deposit nearly all of it's ozone onto fabrics, carpet, etc., keeping the concentration of this chemically-active O3 molecule very low, and thus protecting your tires. The opposite situation, up on a roof rack or in a breezy garage, ages tires at an alarming rate with, or without, direct sunlight.
Another inescapable effect of aging is the weakening of the bond between plies, resulting in the familiar diagonal bulge-out when the overlapping plies de-laminate. I don't know if atmospheric ozone is involved in this breakdown, but there are several different polymers used in a typical clincher, which each containing it's own plasticizer (oils added for increased pliability). Plasticizers leaching out over time into the adjacent materials may affect the properties of each material: tread, sidewall rubber, casing and the bonding agent between plies.
Lastly, humidity is known to readily permeate most polymers and often has a deleterious effect on the material over time.
I face the same problem storing high-quality 27" tires, not knowing which tire will be the next one for which production ceases. 1-1/8" tires measuring 25mm (mounted) are my choice for half the bikes I own. Cheng-Shin C-732 is a decent, traditional, very low cost alternative retailing for about $10 and being molded much like a gumwall but without such thick rubber on the sidewalls. Weight is 290g or so.
David Snyder
Auburn, CA usa
> Anyone have any tips for preserving older clinchers? Armorall?
>
> I've stored tires in black plastic bags with good result. But now I have
> a set of 10 year old Continentals (brown sidewalls) I love, but they're on
> a bike that doesn't get many miles. I'd hate to see them deteriorate
> before I can wear them out, and they're on the only bike they fit.
>
> Mark Stonich;