The bags are actually very tough & 15 years, well, they will go a long, long way while looking aged. Ive even with a couple of cars with the original bags in place. The only bag leak Ive ever had was a new front bag of my 6.3 that went flat on a truck on a transcontinental ride. Pinched a bag so the leak only appeared when inflated. I was rather new to the need for blocks at the time.
I'd suggest focusing on any massive external leaks first, unless replacing the whole system in one pass.
regards
Michael Davies