Justin Bieber, Instagram

Instagram

They grow up so fast!

Birthday boy Justin Bieber has officially joined the 21 club today and can now legally drink in the U.S.

We've seen the superstar singer go from his "Baby" days to his "Boyfriend" moment and now, the Bieb is in his "Heartbreaker" stage as the once little singer is now a man—melting Beliebers' hearts with his sophisticated looks and sexy abs. We hope that his 21st will be his best year yet.

In honor of Bieber's milestone birthday, let's take a look at some of his sexiest shirtless pics. And happy 21st birthday, Justin!

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