Assuming such a de-aging mutation had occurred, the countless other ways to die would eventually kill all such people. Setting aside the fact that - in a long e

Assuming such a de-aging mutation had occurred, the countless other ways to die would eventually kill all such people. Setting aside the fact that - in a long enough time line - an illness, accident, combat, or some external factor independent of one's baseline physical condition would be guaranteed to get you, other natural effects of longevity like e.g. cancer incidence may not be reduced by <insert whatever this hypothetical mutation does> either. Aside from that, it's only in the last millennium or so that we have record keeping that we could plausibly consider credible enough to prove the existence of such a person - in ages before that we have countless myths of immortality or unnatural longevity that we immediately disregard as fictional. So if Methuselah really had lived 969 years due to this mutation, we'd all be none the wiser!