Justin Bieber is hot, ready and legal ... at least according to Rolling Stone magazine.

The 18-year-old "Boyfriend" singer, posing on the cover in a white wifebeater, is flaunting his muscles and giving his best Zoolander face.

As for becoming an adult, Biebs tells the mag:

    "I feel like I carry myself in a more manly way. I don't carry myself as a boy."

The singer's manager Scooter Braun is chiming in on Bieber's relationship with Selena Gomez and how it could lessen his appeal with his young fans. Scooter adds:

    "Personally, I think that's all a bunch of bull***t. Yes, there's gonna be some girls that if they see him with a girlfriend, it kills the dream – but there's also gonna be girls that see him with a girlfriend, hear about the romantic things he does and want him even more."

Baby, baby, baby ... ohhhhh!