Lately its the design that's been wearing out first on kayaks. Unlike my kind of craft, kayaks have been going through quantum leaps for about the last ten years. From what I understand there have been lots of radical improvements that have rendered cutting edge boats obsolete within a couple of years.
It really depends no how they are treated. If they are stored in the sun all winter, the UV's will eat a kayak in only a couple/few years. Also if they are strapped down on their hulls they can develope an "oil can" which is where the bottom of the kayak starts to pop in rather than out. You should try and strap your boat down on its side. These are the biggest things when buying a used boat, so inspect the bottom of the boat and find out where it's been stored.
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