From the NRA's synopsis of Maryland's laws on purchase, possession and carrying of firearms:
It is unlawful for any person to sell or transfer a handgun to any person whom he knows or has reasonable cause to believe is prohibited from possessing a handgun (see "POSSESSION" above) or if the buyer or transferee is:
a. Of unsound mind.
b. Visibly under the influence of alcohol or drugs.
c. Under 21.