A twenty-something is a person in the age group 20 to 29. As in the case of thirty-something, no single word exists in the English language to denote this group or its members, in the manner of quadragenarian to mean a person 40 to 49 years old, quinquagenarian for 50 to 59, and so on.

Beginning in the late 1980s, however, the terms "twentysomething" and "thirtysomething" took on new meanings beyond their ordinary ones, as they were used to convey the idea that persons occupying these respective age brackets at that time in Western societies — particularly the United States — exhibited contrasting collective personalities and worldviews. The practice of using these two terms in this fashion appears to have been set in motion by the title of a popular American television drama named thirtysomething. This show debuted in 1987.

The "thirtysomethings" of the late 1980s were seen as the last of the Baby Boomers, and in addition to the aforementioned television show, such films as The Big Chill were also made about them. The characters in these stories were regarded as differing in many vital respects from younger characters depicted in "youth-genre" films of the previous decade or so up to that point, beginning with Almost Summer (1978) and continuing with later offerings such as Fast Times at Ridgemont High (1982) and The Breakfast Club (1985). This latter group was considered the readership targeted by Douglas Coupland's groundbreaking 1991 novel Generation X: Tales For An Accelerated Culture and became popularly known as Generation X as a result; today, however, the term Generation X is more commonly used to refer to a still younger demographic, who were not yet in their twenties when the word "twentysomething" entered the lexicon, with Coupland's "generation" — those born "in the late 1950s and 1960s" — now often labelled Baby Busters.