The Republic by Plato
Translated by BENJAMIN JOWETT, M.A.,
Late Regius Professor of Greek
University of Oxford
Book V: On Matrimony and Philosophy

A woman, I said, at twenty years of age may begin to bear children to the State, and continue to bear them until forty; a man may begin at five-and-twenty, when he has passed the point at which the pulse of life beats quickest, and continue to beget children until he be fifty-five.