The concept of adolescence:

Although the first use of the word “adolescence” appeared in the 15th century and came from the Latin word “adolescere,” which meant “to grow up or to grow into maturity” (Lerner & Steinberg, 2009, p.1), it wasn’t until 1904 that the first president of the American Psychological Association, G. Stanley Hall, was credited with discovering adolescence (Henig, 2010, p. 4). In his study entitled "Adolescence," he described this new developmental phase that came about due to social changes at the turn of the 20th century. Because of the influence of Child Labor Laws and universal education, youth had newfound time in their teenage years when the responsibilities of adulthood were not forced upon them as quickly as in the past. http://www.massculturalcouncil.org/services/BYAEP_History.asp

With the trend towards an expectation of college education, we will need an extended concept to include the early twenties.

Edit: "Emerging adulthood is a phase of the life span between adolescence and full-fledged adulthood, proposed by Jeffrey Arnett in a 2000 article in the American Psychologist."