Justin Bieber -- I'm a RESPONSIBLE 18-Year-Old

Justin Bieber insists ... he ain't some out-of-control pot-smoking sizzurp-chugging dumbass -- despite several photos circulating which suggest otherwise -- he's just a "responsible" 18-year-old who's enjoying life.

Bieber appeared on Jimmy Fallon last night, when the late night host joked about police pulling Bieber's white Ferrari over ... a ludicrous number of times.

Bieber responded, "I don't do anything. I promise. It looks bad. You see pictures of me getting pulled over, but really the tints are really dark ... I need dark tints because I don't want to cause accidents because people want to take pictures. I just thought it would be more safe for everyone else if I had dark tints."

He added, "I'm thinking about other people!"