Bob is right, quality and care are key factors. The oldest item of gear to have been continuously in my possession is a British-made Typhoon Super Star mask from the early 1960s:
Because it was a birthday or Christmas present from my parents when I was a schoolboy, it has huge sentimental value and I've never even snorkelled with it. When I joined the sub-aqua club at university, I bought a French-made compensator mask to train with, which has long gone.
Recently I purchased a set of 1960s-vintage Typhoon fins, mask and snorkel on eBay. The unused set came in a box, which had been stored in the loft of a house:
The natural rubber parts of the fins, mask and snorkel are all as soft and supple as they would have been on the day they were bought during the 1960s. The mask was another Super Star model and its condition was identical to mine with a soft, supple skirt and strap and with unblemished stainless steel rim and buckles. In the case of the Typhoon breathing tube, I was particularly impressed by the absence of perishing or hardening of the mouthpiece, keeper and splash cap and by the shiny state of the anodised aluminium barrel. Sadly, though in perfect condition, the Juniormaster fins are too small to fit my oversized feet. That such coloured rubber underwater swimming gear has lasted so long is ample testimony to the quality of some diving equipment manufacturers' products in the mid twentieth century and its condition flies in the face of the doom-laden warnings back then in diving manuals to the effect that all coloured gear will inevitably perish and/or harden sooner or later, which is clearly not the case with my British Typhoon collection above.