Prior to the nineteenth century, there was insufficient evidence to make knowable claims either for or against centenarian longevity.[6] Even today, no fixed theoretical limit to human longevity is apparent.[7] Studies in the biodemography of human longevity indicate a late-life mortality deceleration law: that death rates level off at advanced ages to a late-life mortality plateau. That is, there is no fixed upper limit to human longevity, or fixed maximal human lifespan.[8] This law was first quantified in 1939, when researchers found that the one-year probability of death at advanced age asymptotically approaches a limit of 44% for women and 54% for men.

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