19. Akdong Musician: 21 Under 21 (2014)

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Birthdates: Lee Chan-hyuk: Sept. 12, 1996 (Age: 18); Lee Soo-hyun: May 4, 1999 (Age: 15)

Last Year's Rank: N/A

Why They're Hot: A year after winning Korea's American Idol-esque competition K-pop Star 2, brother-sister duo Akdong Musician hit the scene with their debut full-length Play in April 2014. Produced, composed and written entirely by 18-year-old Chan-hyuk, the harmony-heavy folk-pop album saw immediate domestic success with the ultra-catchy lead single "200%,” which ruled at No. 1 for two weeks on the K-Pop Hot 100. The duo also earned top 10 hits with their second and third singles, the peppy "Give Love” (No. 3) and melancholic ballad "Melted” (No. 8). And Play felt love stateside, hitting No. 20 on Billboard's Heatseekers Albums chart -- a rare feat for a new K-pop act -- and snagging a No. 2 slot on World Albums.

21 Under 21's Next Class | Top 21 Songs by Under-21 Singers 

Billboard's 2014 21 Under 21 List