No. 1. Justin Bieber
Why He's Hot: Any way you slice it, Justin Bieber is at the top of the underage set, and in 2011 he gracefully takes our 21 Under 21 crown from now-21-year-old Taylor Swift. Although the Biebs hasn't released a proper album since "My World 2.0" in March 2010, the pop superstar has spent the past year essentially releasing everything else imaginable: an acoustic album, a 3D movie, a memoir, a new fragrance, a singing doll, and a lock of his hair, which became a hot item on eBay. Next up is a Christmas release, an insanely anticipated new album, and more time spent with pop star girlfriend Selena Gomez in between award shows and sold-out arena concerts. It's Justin Bieber's world -- we're just living in it.
Video above: Justin Bieber talks at the 2011 Billboard Music Awards