^ Good answers. I concur that as long as a racket has not been abused (exposed to temperature extremes or strung well over the recommended limit, physically abused, etc.), it should continue to play well for years and years. The exception is the occasional defective frame that is prone to cracking or snapping most of which would have died at a young age.
I occasionally mentally abuse some of my rackets (yell at them, lock them in a closet, etc.) but that does not seem to have an adverse effect on their playing characteristics.
If I wait long enough, my graphite rackets with morph into diamonds?
(Actually, Gamma used to have a diamond fiber series a few years back.)