Entertainment

Tweeny-bit grown up

On Friday, Justin Bieber belted out his new hit single “Boyfriend” for the 8,000 screaming pre-pubescent girls who packed into Rockefeller Center for his “Today” show concert.

He was smirking and winking his way through the Justin Timberlake-esque number, when, suddenly, Bieber reached down and gave his white pants a little tug. The teeny boppers hooted with delight. The mothers seemed surprisingly pleased, too.

And with that single crotch grab, the message was clear: Bieber was a “baby” no more.

Ladies, meet the older, taller, decidedly sexier Bieber.

The 18-year-old Canadian crooner has shed his purple duds, boy-band hair flop, saccharine beats and kissing coyness for James Dean threads, a side-swept spiked mane, songs about baby-mama drama, and PDA galore with his Disney star girlfriend, Selena Gomez.

“Our conversations are much different now,” says Ryan Good, Bieber’s “swagger coach” (read: stylist) and road manager. “Before, when he was a kid, I was always giving him advice. Now he gives me advice. We can talk about family, friends, relationships — a brother-to-brother conversation.”

Indeed, Bieber’s life is reaching a whole new realm of celebrity. That of the sex figure. Don’t “belieb” it?

Look no further than the album’s final track, “Maria” — an angry missive to Mariah Yeater, who claimed a then-16-year-old Bieber fathered her child during a 30-second backstage romp at an LA concert in 2010.

“Maria” stands out on the record, which, despite its clubby beats, mainly boasts lyrics about fondue and how love changes the world.

Because even though he has 21 million Twitter followers and $108 million in personal earnings, Bieber’s main objective is to keep a die-hard bunch of 12-year-olds happy.

Which is why Bieber didn’t prematurely dive into the big kids’ pool.

“Justin’s not a man yet. And I don’t think his constituency is grown women; the majority of his fans are younger than he is, and he’s still trying to appeal to them. That was a very smart decision, to keep with his fan base,” says Bieber’s longtime vocal coach, Jan Smith, a k a Mama J.

That’s not to say there’s not a struggle.

“There’s that fine line when you’re about to turn 18 . . . and your fan base is teenage girls and you deep down inside want to make that transition where the college kids like you,” says Rodney Jerkins, one of the producers of “Believe.” “And that’s a challenge, that’s a real challenge.”