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The term middle-aged, invented by Descartes, comes from the Latin, medeus, meaning 'not really old' and ageis, meaning 'if you look at it in a certain way.

Marilyn Suzanne Miller (2007). “How to Be a Middle-Aged Babe”, p.4, Simon and Schuster
The term middle-aged, invented by Descartes, comes from the Latin, medeus, meaning 'not really old' and ageis, meaning 'if you look at it in a certain way.