S.C. Code Regs. § § 38-424
The rules and regulations adopted by the United States Department of Transportation relating to safety of operation and to equipment (49 CFR Parts 382, 383, 385, 387, and 390-399 and amendments thereto), and the rules and regulations adopted by the United States Department of Transportation relating to hazardous materials (49 CFR Parts 107 and 171-180 and amendments thereto) shall apply to all motor carrier vehicles engaged in interstate commerce and intrastate commerce over the highways within the State of South Carolina, whether common carriers, contract carriers, exempt carriers, or private carriers, except where these aforementioned rules and regulations may conflict with South Carolina Law, provided that any rule or regulation which fixes the minimum age of vehicle drivers at 21 years is hereby changed insofar as South Carolina intrastate carriers are concerned to establish a minimum age of 18 years for vehicle drivers.
S.C. Code Regs. § 38-424