21. Fifth Harmony: 21 Under 21 (2013)

Fifth Harmony

Last Year's Rank: N/A

Birthdates: Allyson Brooke Hernandez - born July 7, 1993 (age 20), Normani Kordei Hamilton - born May 31, 1996 (age 17), Lauren Michelle Jauregui - born June 27, 1996 (age 17), Karla Camila Cabello - born March 3, 1997 (age 16), Dinah Jane Hansen - born June 22, 1997 (age 16)

Why They're Hot: Formed under the blessing of Simon Cowell on last year's "X Factor," Fifth Harmony's brief career so far is on the fast track for success. Their triumphant debut single "Miss Movin On," produced by the Suspex (the duo behind Demi Lovato's "Heart Attack"), became the first track from an American "X Factor" alumni to break the Hot 100, peaking at No. 76. The song samples a debut EP expected Oct. 22 via Epic Records/Syco Music. In the meantime, the quintet is on tour with Cher Lloyd and was recently featured in Disney's "Next Big Thing" series.