Justin Bieber -- I'm Not a Man... I'm Just A Boy

Justin Bieber is whining about being Justin Bieber ... and even though he's been doing everything he can to be seen as a grown-up, he now claims he's just a misunderstood "19-year-old-boy."

JB was on the radio with Ryan Seacrest this morning to plug his new documentary, "Justin Bieber's Believe" ... which he claims will show how difficult it's been for him to go from teen star to manhood in the public spotlight.

That's odd, talking about becoming a man ... while complaining he's just a boy? Can't have it both ways, Biebs.