Actually if that site is right you need to be 18 to buy a long gun
from a dealer by federal law
"Dealers may not sell or deliver a long gun, or ammunition for a long
gun, to any person the dealer knows or has reasonable cause to believe
is under age 18".
Since you need to show ID and fill out a 4473, that is pretty certain.
That's what I said.
A buyer must be 18 to purchase a long gun in Maryland.