The drinking age in the United States is 21. Most restaurants that serve alcoholic beverages allow those under 21 inside. Generally speaking, if only those over 21 are allowed inside, you will be greeted at the door by someone asking to see identification. These people are often poorly trained and anything other than a state driver’s license may confuse them, so be forewarned.
Drunk driving is a problem on the highways of America, and South Carolina  is no exception. Always drive defensively, especially late at night, and obey all posted speed limits and road signs—and never assume the other driver will do the same. You may never drive with an opened alcoholic beverage in the car, even if it belongs to a passenger.
As far as retail purchase goes, generally speaking in South Carolina you may only buy beer and wine on Sundays, not hard liquor. Closing times at bars generally vary by municipality. As a general rule, the closer you are to the coast the more lax the attitude toward drinking.