Professional cleaning & inspection
I have mine cleaned, inspected, stored, and (if needed) repaired each season by a local sail loft. It's the small repairs they catch & fix before becoming big that can extend the working life. UV & wear appear to work on the threads & seams first.However, on a limited budget, if you clean them yourself just make sure they are rinsed thoroughly in fresh water to remove salt, detergent, bird poop, whatever. And, make sure they are thoroughly dried before folding & storing or you'll have serious mildew. I suggest you should try to schedule those old sails in for a professional job at least once every three years.A final note -- using 10+ year old sails is OK, but just remember thy are probably a lot weaker than new ones and can't take heavy use. Be tender with them! I used a rental boat a few years ago in a brisk wind & chop and the main suddenly split in half horizontally right across the reef line as I was tacking. The falling boom barely missed my head. The owner later admitted the sails were 18 years old!