I've had a lot of experience with older papers, bromide papers keep remarkably well in general, as Simon says contrast & speed drops with age but in my case 1960's Kodak Bromide Paper and Ilfobrom there was no increse in base fog.
Warmtone papers age very much faster, there's a detectable shift in image colour after a year, but that's only when trying to coax maximum warmth from a paper.
Some warm tone papers keep very poorly, Sterling (made in India) was the worst, one of the old Oriental WT papers wasn't much better. I'm talking less than 5 years.
Variable contrast papers differ quite markedly, in general Ilford keep better than Kodak.