Justin Bieber & Madison Beer -- JB Inches Dangerously Close to 15-Year-Old's Boob

Our Bieber sources say Justin has been like a big brother to Madison for 2 years, and this was meant to be nothing more than a sweet photo of 2 people who have grown close. They say Justin has helped mentor Madison in her singing career.

Underage girls are all the rage -- days after 13-year-old Willow Smith posed in bed with a shirtless older man ... 20-year-old Justin Bieber came within inches of mashing his face into a 15-year-old girl's breast.

Bieber posted the photo of himself Thursday night, posing with up-and-coming singer Madison Beer and her left boob. Any closer and he might get himself in trouble.

You'll recall, Willow Smith ruffled some feathers with a recent Instagram photo, showing her lying in bed with shirtless 20-year-old actor Moises Arias.

The irony: Bieber's too old for Beer, but too young for beer.