Tire Aging

Do tires get old? I have a 96 XJ with a 4 1/2 inch lift. I'm running on BFG All-Terrain TK's, 32 x 11.50 R15 on AR 39 Aluminum rims. Lately, I've been losing pressure on the trail. I usually air down to 18 psi. Running the streets, I run 32 psi. I've only had this air loss problem for less than a year. Here's the kicker. I only have the problem on the rear tires and spare, never on the front. The three problem tires are at least 8 years old. The two on the front are no more than a year old. (Had to replace them because of sidewall cuts.)
Is it possible that the bead rubber has stiffenned so that it won't hold pressure? Or is it just a coincidence. I have experienced pressure loss even on relatively mild trail riding.
Are there ways to improve the seal. I've already taken them apart and cleaned the bead area before the last two rides and still had the problem.

Yes,tires get old,(all tires even new lose air thru the rubber),you could try using nitrogen it doesnt expand/contract as much with temperature and the molecules are larger so they have a harder time "getting" out!

How fast do they lose air? If they lose air relatively quickly then it could be a number of things: a Nail thats not visible, valve stem/core, bead. best bet is to have them checked out @ a tire shop. I work @ America's Tire Co. and we have a tank that we dunk tires in to locate leaks. If it is the bead it is a good idea to clean it as you did but applying bead sealer helps even further. If they lose air very slowly than you just need to put up with it or get new tires.

tires get old. the rubber dries out and gets weak, and you will lose air thru the rubber, not just at the bead. they usually last about 4-5 years at most.