"Categorical age", if you'll allow me to coin a term for this phenomenon (infancy, childhood, adolescence, middle age, ... - your conception may vary) is more or less a qualification of our personal relationship to time and mortality. So while we might have broad consensus that a 5 year old is a child and a 70 year old is a senior, it's not determined by numerical age. Just a near 1 correlation.

After all, we never know our numerical middle age except in retrospect.