1. One Direction: 21 Under 21 (2013)

One Direction

Last Year's Rank: 2

Birthdates: Zayn Malik - Jan. 12th, 1993 (age 20); Harry Styles - Feb. 1, 1994 (age 19); Louis Tomlinson - Dec. 24, 1991 (age 21); Niall Horan - Sept. 13, 1993 (age 20); Liam Payne - Aug. 29, 1993 (age 20)

Why They're Hot: Success has never been a stranger to the dreamy dudes of One Direction – the band's star has burned bright ever since the English-Irish group graduated from British ‘X Factor' and emerged onto the worldwide music scene three years ago. But over the past 12 months, the lads have exploded with the force of a supernova, becoming an inescapable pop phenomenon that's hot enough to incinerate any obstacle in its universe. The numbers speak for themselves: In November 2012, One Direction saw their sophomore release "Take Me Home" top the charts in in countries around the world and became the only U.K. group in history to have their first two albums debut at no. 1 on the Billboard 200 chart. In 2013, 1D placed an impressive six songs on the Billboard Hot 100 chart (more than any other group this year). One Direction upped the ante in August, when their spunky pop-rock single "Best Song Ever" debuted at no. 2 on the Hot 100 and became the group's highest charting hit yet. That same month, 1D released "This Is Us," a behind-the-scenes, 3D concert spectacular that has grossed more than $62 million to date, making it one of the biggest concert films of all time. And even though the world is already at their feet, the fantastic five have no intention of slowing their roll. Before year's end, One Direction will return with "Midnight Memories," its anticipated third album, and also plans to embark on yet another world tour in 2014. Sadly, all but one member of One Direction will be of legal drinking age by the time we do next year's 21 Under 21 tally. But until then, the guys are dead set on following their own advice and living while they're young.