There might be something to that, I can't say for sure. I can say that a 15 year old water heater has got to be about ready for the retirement home. You could try flushing and see what happens. On other hand, if it's working fine as it is, what is there to gain by flushing it at this point? If it was mine, I'd leave well enough alone but watch it closely for signs of leaking, and I would certainly budget for a new heater in the near future.