18. Mindless Behavior: 21 Under 21 (2013)
Last Year's Rank: 13
Birthdates: Prodigy - Dec. 26, 1996; Ray Ray - Jan. 6, 1997; Princeton - April 21, 1997; Roc Royal - July 23, 1997 (All age 16)
Why They're Hot: The swaggy LA-based boy band scored its first chart-topper this year, as sophomore album "All Around the World," debuted at No. 1 on R&B Hip-Hop Albums and moved a personal best 37,000 copies that week as well, according to Nielsen SoundScan. They also got acquainted with the Billboard team, stopping by the Billboard studio to talk about their new single "All Around the World," and complete a "How Well Do You Know Your Bandmates?" session. The first US headlining tour was chronicled in the documentary "Mindless Behavior: All Around the World."