This video should be rated "R" for Rats!!! ... which is what 13-year-old girls will say when they watch Justin Bieber kiss Selena Gomez after a romantic karaoke duet Sunday night in L.A.
Justin and Selena hit up Lucky Strike, Downtown -- where they performed their own rendition of the Kid Rock/Sheryl Crow duet, "Picture."
But their curfew must have been pretty strict -- because J&S didn't linger ... hitting the road before midnight.
Oh, to be young ... and rich. Screw young. Rich.