I don't change mine until they get punctured. They do make "heavy duty" tubes that have thicker rubber. I only use those if I plan on racing in the desert. I suppose a tube could "rot", mine never hold air long enough for that to happen!
Ultra heavy duty tubes will give you much more flat protection and you can keep the stockers as spares. You were looking for an excuse to buy new stuff right ? ;)
Experience says to use heavy duty tubes for heavy off road use (away from the mx tracks). There is nothing wrong per say with the stock ones, but they are much lighter. The lighter has always been asking for trouble a long way from home. Yes, heavy duty tubes will last for several tires. Just my .02. Cheers John
I think I would go ahead and change them if they're four years old. Rocky Mt ATV has MSR heavy duty tubes for around 10.00. If you'll get your new tires from there, you'll save enough $$$ to pay for new tubes.
This is true if you change your own.... I have bent 2 tire iron to the unuseable state, (Darn Maxxis 6001's) that I have mine changed now. Usually if you buy tires locally, they change them for almost nothing if they want your business, Otherwise I have spent almost $60 to have 2 changed that I bought from Rockymountainmc.com ... never again! :|