No. 13. Cody Simpson
Why He's Hot: Australian teen star Cody Simpson is slowly becoming the all-encompassing pop phenom that could make him a household name by this time next year. After working with Flo Rida on his debut EP, "4 U," last year, the singer-songwriter unveiled a "very emotional" six-song EP, "Coast to Coast," in September; its first single, "On My Mind," has already earned 7 million views on YouTube. With a full-length debut expected for early 2012, Simpson may move into the highest tier of our 21 Under 21 list in 2012.