I understand but for it to be discrimination, and I could be wrong, they would have to bar anyone of an age group from entering or purchasing anything.
By definition, it's discrimination according to MD law, since DSG is barring everyone 18-20 from making certain legal purchases. DSG can refuse sales to specific individuals for non-discriminatory reasons but cannot wholesale refuse everyone 18-20 years old. MD law makes no distinction about age, presumably to cover discrimination exactly like DSG.