Newton’s mathematical creativity had ended by age forty.

Evidence

Newton gave up mathematics at fifty, and had lost his enthusiasm long before; he had recognized no doubt by the time that he was forty that his great creative days were over. — G. H. Hardy, A Mathematician’s Apology, 142

Categorization

• corroborated: 2 independent passages support this claim — G. H. Hardy, A Mathematician’s Apology, 142