From "Star Trek: Requiem for Methuselah (#3.19)"
[I was born
] in that region of Earth later called Mesopotamia, in the year 3834 B.C., as the millennia are reckoned. I was Akharin, a soldier, a bully and a fool. I fell in battle, pierced to the heart - and did not die.
] Instant tissue regeneration, coupled with some perfect form of biological renewal - you learned that you were immortal...
...and to conceal it, to live some portion of a life, to pretend to age and then move on before my nature was suspected.