aet.

late 13c., "long but indefinite period in human history," from Old French aage, eage (12c., Modern French âge) "age; life, lifetime, lifespan; maturity," earlier edage (11c.), from Vulgar Latin *aetaticum (source also of Spanish edad, Italian eta, Portuguese idade "age"), extended form of Latin aetatem (nominative aetas), "period of life, age, lifetime, years," from aevum "lifetime, eternity, age" (from PIE root *aiw- "vital force, life; long life, eternity").

Expelled native eld (Old English eald) "old age; an age; age as a period of life." The meaning "time something has lived, particular length or stage of life" is from early 14c. Used especially for "old age" since early 14c.; the meaning "effects of old age" (feebleness, senility, etc.) is from mid-15c.

In geology, in reference to great periods in the history of the earth, by 1855; in archaeology, from 1865 (Stone Age, etc.) naming periods for the materials used to make weapons and tools. Sometimes in early modern English "a century" (similar to French siècle "century," literally "an age"), hence plural use in Dark Ages, Middle Ages. To act (one's) age "behave with appropriate maturity" is attested by 1927.