Why is the ninth month called September?
1829 as the years of someone's life between 20 and 29; 1830 as a decade of years in a given century. See twenty.
Old English twentig "group of twenty," from twegen "two" (see two) + -tig "group of ten" (see -ty (1)). Cognate with Old Frisian twintich, Dutch twintig, Old High German zweinzug, German zwanzig. Gothic twai tigjus is even more transparent: literally "two tens."
The card game twenty-one (1790) is from French vingt-et-un (1781). Twenty-twenty hindsight is first recorded 1962, a figurative use of the Snellen fraction for normal visual acuity, expressed in feet. The guessing game of twenty questions is recorded from 1786. The Twentieth Century Limited was an express train from New York to Chicago 1902-1967.