Robin Thicke -- I Buy the Booze 'Cause My GF's Too Young

Robin Thicke learned one of the drawbacks to dating a 20-year-old model ... you can't send her out for booze.

The crooner and his smoking hot GF, April Love Geary, hit up Gil Turner's on the Sunset Strip Tuesday night ... one of the most famous liquor stores in town.

Lots of stars buy the hard stuff at Gil Turner's, but many wait in the car while their less famous partners go inside. That option is not currently available to Robin.

Small price to pay ...