vicenarian
A person whose age is between twenty and twenty-nine years, inclusive; someone in their twenties.
Melissa is a vicenarian at twenty-one years of age and a hairdresser.
Where it comes from
From vicenary + -ian, from Latin vīcēnārius (“vicenary”), from vīcēnus (“twenty each”) + -ārius (“-ary: forming adjectives and related nouns”).
Adapted from the Wiktionary entry vicenarian (CC BY-SA 4.0).