Entries linking to underage
Old English under (prep.) "beneath, among, before, in the presence of, in subjection to, under the rule of, by means of," also, as an adverb, "beneath, below, underneath," expressing position with reference to that which is above, from Proto-Germanic *under- (source also of Old Frisian under, Dutch onder, Old High German untar, German unter, Old Norse undir, Gothic undar), from PIE *ndher- "under" (source also of Sanskrit adhah "below;" Avestan athara- "lower;" Latin infernus "lower," infra "below").
Productive as a prefix in Old English, as in German and Scandinavian (often forming words modeled on Latin ones in sub-). Notion of "inferior in rank, position, etc." was present in Old English. With reference to standards, "less than in age, price, value," etc., late 14c. As an adjective, "lower in position; lower in rank or degree" from 13c. Also used in Old English as a preposition meaning "between, among," as still in under these circumstances, etc. (though this may be an entirely separate root; see understand).
Under the weather "indisposed" is from 1810. Under the table is from 1913 in the sense of "very drunk," 1940s in sense of "illegal" (under-board "dishonest" is from c. 1600). To keep something under (one's) hat "secret" is from 1885; to have something under (one's) nose "in plain sight" is from 1540s; to speak under (one's) breath "in a low voice" is attested from 1832.
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." Meaning "time something has lived, particular length or stage of life" is from early 14c. Used especially for "old age" since early 14c.; 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, 1855; in archaeology, from 1865 (Stone Age, etc.) naming periods for the materials employed for 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.