I have a set of ProKennex silver ace 90's, I think they were made in the 80's and they still play great. No real problems with stringing, it is a fiberglass/graphite composite frame. Graphite frames will start to break down eventually if you continually string it and unstring it, as well as just leaving it exposed to the free elements. From my experience, racquets should last a really long time as long as they are taken care of. Answering your last question, a freshly made racquet vs a racquet with age will play slightly different, the freshly made one being stiffer than the aged one.
A little extra information: Graphite is a structure of carbon atoms bonded to each other, Diamonds are also made of carbon atoms bonded to each other, the differences between the two are in the structure. The Diamond is a dense lattice structure, which is why it is harder than graphite. Those carbon atoms will decay over time(in diamonds it will take most likely thousands of years before they become like graphite). So graphite should live longer than the human lifespan, unless of course you're a cybernetic being or an immortal.