CIRENCESTER - AUGUST 05: Prince William, Duke of Cambridge attends the Jerudong Trophy at Cirencester Park Polo Club on Augus
Anwar Hussein / Steve Granitz

A thing Justin Bieber might not know first-hand yet is that when men get older, many of them lose their hair. Another thing Justin Bieber might not know yet is that this is generally something people don't talk about. 

Good thing Bieber's rich enough to buy the common courtesy he seems to be lacking. The "As Long As You Love Me" singer had some pretty rude things to say about Prince William and his thinning hairline, and we're guessing his mom's somewhere shaking her head right now. In an interview with U.K.'s Rollercoaster magazine, Bieber said, "I mean, there are things to prevent [balding] nowadays, like Propecia. I don't know why [Prince William] doesn't just get those things, those products. You just take Propecia and your hair grows back. Have you not got it over here?" Damn. Dissin' William and England in the same paragraph!

This situation is kind of like when you're talking about your mom being crazy, and then your friend is like, "Oh, totally," and then you're like, "Wait, are you calling my mom crazy?" Off limits, bro. Prince William owns a mirror or two—he knows what's going on upstairs. He doesn't need your commentary about it, Biebs! Stick to making videos where you get beat up!