How do SUPs age and what is their lifespan, let's say carbon production model of top brand. Mine accumulate minor dings/cracks from paddle contact, absorb some water before I notice and repair, dunno if it can fully dry at repair shop. A couple of my non-carbon boards have pressure dents on bottom presumably from air travel. So assuming board doesn't get damaged bad enough to require a major repair, how long can they last and does their performance decline? Asking in part cause I see a used board that interests me, but it's ~4y old, not sure what fraction of original value it should hold, e.g. if it appears in good condition but has 4y of constant use.

I don't think you can assess board condition based on age. There are so many factors in play. Eg- brand, frequency of use, storage conditions, transport (is it thrown in the ute after a surf to bounce around or put in a bag inside the car), number of overseas trips, beach conditions (rocky or sandy), surf conditions (constant choppy beach break or glassy point with a smooth paddle back), climate, has it been knee paddled, attention to maintenance and repairs

Deckpads can lose some grip with age. Key factors for me would be weight, no major repairs, open dings or poor repairs, no soft spots (delam).non-delaminated dents are nothing if they are small. I would replace gore Tex vent on used board if possible. I've beaten a few boards into the ground now and the only things that affected the performance were weight from repairs or breaking in half. I had a board creased on bottom but not rails and it was still good after repair.

Thanks for the insights

I've had issues with osmosis on two boards now, both good brands

Brought this 8-10 speed back from the dead this board is 10+ years old damaged the nose and had to chop and reshape after that the bottom half delaminated fixed that ,still is light and surfs like the day I bought it.Impossible to sell so why not keep riding with current board prices sky high it makes sense.

Yeah my 94L Speed has major repairs and was delaminating, but is currently my favorite board in quiver. I paid only 300 USD for it, where it gets tricky is the used, decent looking boards that are $1-1.5k, half the price of new, clearly a deal if someone bought wrong board needs to resell, but what if it looks ok but you know it's been surfed every week for several years. Dave Boehne mentioned boards not floating as well after aging, but maybe a plus if you're not already on a sinker.