21 Under 21: One Direction (2011)
21 Under 21: One Direction (2011)

No. 19. One Direction

Birthdates: Zayn Malik - Jan. 12th, 1993 (age 18); Harry Styles - Feb. 1, 1994 (age 17); Louis Tomlinson - Dec. 24, 1991 (age 19); Niall Horan - Sept. 13, 1993 (age 18); Liam Payne - Aug. 29, 1993 (age 18)

Why They're Hot: After a third place finish on the seventh season of the British 'X Factor,' boyband One Direction signed to Syco/Sony and quickly began ruling the U.K. singles chart with their No. 1 smash "What Makes You Beautiful," which scored the biggest first-week sales of 2011, according to the Official Charts Co. Formed on the suggestion of Pussycat Doll Nicole Scherzinger, the British-Irish quintet of Liam Payne, Louis Tomlinson, Niall Horan, Harry Styles and Zayn Malik elicit the kind of rampant fandom seldom seem outside of Bieber appearances. The band's admirers aren't limited to just squealing teen girls, however; BBC Radio exec Nigel Harding describes "Beautiful" as a track that "succeeds musically and lyrically … and could lead to One Direction's U.K chart performance being repeated elsewhere in the world."

NEXT: Porter Robinson