No. 1: Justin Bieber
Birthdate: March 1, 1994 (Age 18)
Why He's Hot: Fending off the boys of One Direction was no easy feat, but Justin Bieber returns to the top of our 21 Under 21 list this year. Since earning last year's crown, Bieber has snagged two more No. 1 albums -- the holiday set "Under the Mistletoe" and the more adult-leaning full-length "Believe" -- and converted more Beliebers with slick award-show performances, radio hits like "Boyfriend" and "As Long As You Love Me," tabloid-ready pictures with girlfriend Selena Gomez and those adorable, now-legal good looks. As he prepares to kick off a massive arena tour in support of "Believe," Bieber remains more powerful in the pop universe than ever; let's not forget that the Biebs unleashed Carly Rae Jepsen's "Call Me Maybe" upon the United States with a single tweet last winter. He's not a boy, not yet a man, but right now, Justin Bieber is the supernova of teen pop stars with charisma, popularity and swagger to spare. Once again, it's Biebs' world, and we're lucky to live in it.
Video Below: Justin Bieber performs "Boyfriend" at the 2012 Billboard Music Awards