Justin Bieber is "definitely not yet a man," according to his manager, Scooter Braun.

The 18-year-old singer has showcased a new, more mature sound on his third album, 'Believe', but Scooter says he's still got a long way to go before he's fully-grown.

He told Billboard magazine: "Adult artist? Just because he's legal now doesn't mean he's an adult.

"He still needs guidance; he's still finding his way. He's no longer a boy, but he's definitely not yet a man."

Scooter added he is "proud" of the 'Boyfriend' hitmaker because he has grown up very fast and in the full glare of the public eye.

He added: "No one in the history of the world has ever grown up with the pressure that he has, being a solo artist that young, that famous, in a world with technology that exposes us 24/7.

"He was literally going through puberty in front of the entire world, with everyone expecting something from him and a lot people waiting for him to screw up.

"He's a fighter. I'm proud of him -- how's he's stayed true to himself and taken control of the entire situation."

Justin - who is dating fellow teen star Selena Gomez - has recently said he finds it hard to trust most of the people he meets, because everyone wants something from him.

He added: "I mean, I keep my guard up a lot, because you know, you can't trust anyone in this business. That's what's sad. You can't trust anybody. I learned the hard way."