Lewis & Short

vīcēnārĭus, a, um, adj. [viceni], of or belonging to the number twenty, vicenary.

  1. I. Adj.: annorum lex me perdit quina vicenaria: metuunt credere omnes, i. e. the law by which young people under five-and-twenty were incapable of making contracts, Plaut. Ps. 1, 3, 69 (Ritschl, quinavicenaria): fistula, twenty quarter-digits in diameter, Vitr. 8, 7; Front. Aquaed. 30; Pall. Aug. 12.
  2. II. Subst.: vīcēnārĭus, ii, m., a youth of twenty, Arn. 2, 58.