13. Bridgit Mendler: 21 Under 21 (2013)

Miranda Penn Turin

Bridgit Mendler

Last Year's Rank: 16

Birthdate: December 18, 1992 (Age 20)

Why She's Hot: "Ready or not, here I come," goes the chorus of Mendler's Fugees-inspired single "Ready or Not," summing up her entrance into pop music. That song marked the highest peak in the Hot 100 (No. 49) to date for the pop singer/Disney actress, who released her debut album "Hello My Name Is…" last year via Hollywood Records. Previously, Ms. Mendler broke onto the Billboard 200 with the soundtrack for her film "Lemonade Mouth" (peaking at No. 4), in which she plays the lead singer of a fictional band. The recent Billboard tastemakers selection released a live concert EP this past April and has spent much of the year touring and working on the next season of the Disney channel series "Good Luck Charlie."