Crib mattresses don't have an expiration date, so their longevity depends on the mattress' quality, durability, condition, and appearance. If you have a used mattress you are thinking of using, inspect it carefully for indents, sagging, broken seams, holes, mold, bugs, or any other issue that would make it unsuitable for reuse. If it is questionable, err on the side of caution and buy one that meets all current safety standards.