Aluminum fatigues, steel may or may not fatigue, carbon fiber breaks. In each case, it's a design issue of dealing with the material, not just some material flaw. I sure wouldn't worry about a "shelf life".
Bare aluminum will develop little pockmarks of corrosion, just like bare steel will rust, and maybe somebody is deducing a shelf life from the bare material or something flaky like that. But that's an issue of how long it'll loook pretty enough to sell, not related to be useable.
Bit of trivia: I was scanning a book at the library on the Tour de France. It showed one guy fixing a flat, and mentioned that he had been thrown out of the TdeF two years in a row by broken forks. That was back in the 1920's.
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"be careful this rando stuff is addictive and dan's the 'pusher'."