sexagenarian (n.)

one more than five; twice three; the number which is one more than five; a symbol representing this number;" Old English siex, six, seox, sex, from Proto-Germanic *seks (source also of Old Saxon and Danish seks, Old Norse, Swedish, and Old Frisian sex, Middle Dutch sesse, Dutch zes, Old High German sehs, German sechs, Gothic saihs), from PIE *s(w)eks (source also of Sanskrit sas, Avestan kshvash, Persian shash, Greek hex, Latin sex, Old Church Slavonic sesti, Polish sześć, Russian shesti, Lithuanian šeši, Old Irish se, Welsh chwech).

As "playing card with six spots or pips" by 1590s. Six-footer "person measuring six feet or more" is by 1828. Six-shooter, usually a revolver with six chambers (firing six shots in succession), is attested from 1842; six-pack of beverage containers is from 1952, of abdominal muscles by 1995.

[Married] In Dana, [Mass.], on the 30th ult. by Ephraim Whipple, Esq. Mr. Jason W. Williams, M. D. to Miss Malinda Chamlin, of Dana,—a couple of six footers. Printers in New York, Ohio, Alabama, Vermont, Connecticut, Rhode-Island, New Hampshire and Maine, are requested to publish the same marriage, that his brothers and sisters may have due notice thereof and rejoice accordingly. [Burlington, Vt., Weekly Free Press, Feb. 8, 1828]

Six of one and half-a-dozen of the other "little difference" is recorded from 1833. Six-figure (adj.) in reference to hundreds of thousands (of dollars, etc.) is from 1840. Six feet under "dead" is from 1942.

The phrase at sixes and sevens originally was "hazarding all one's chances," first in Chaucer, perhaps from dicing (the original form was on six and seven); it could be a corruption of at cinque and sice "exposed to great risk" (1530s), literally "at five and six," using the French names (which were common in Middle English) for the highest numbers on the dice. The meaning "at odds, in disagreement or confusion" in the exact phrase is from 1785; in Middle English the phrase set at (or on) six and seven meant "play havoc, create an uproar."

Proto-Indo-European root meaning "ten."

It forms all or part of: cent; centenarian; centenary; centi-; centime; centurion; century; centennial; cinquecento; dean; deca-; decade; decagon; Decalogue; Decameron; decapod; decathlon; December; decennial; deci-; decile; decimal; decimate; decimation; decuple; decussate; denarius; denier (n.) "French coin;" dicker; dime; dinar; doyen; dozen; duodecimal; duodecimo; eighteen; fifteen; fourteen; hecatomb; hendeca-; hundred; icosahedron; nineteen; nonagenarian; octogenarian; Pentecost; percent; quattrocento; Septuagint; sexagenarian; seventeen; sixteen; ten; tenth; thirteen; thousand; tithe; -ty (1).

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dasa, Avestan dasa, Armenian tasn, Greek deka, Latin decem (source of Spanish diez, French dix), Old Church Slavonic deseti, Lithuanian dešimt, Old Irish deich, Breton dek, Welsh deg, Albanian djetu, Old English ten, Old High German zehan, Gothic taihun "ten."