In 48 states across the country, someone who is 18 years old can receive their commercial drivers license (CDL), assuming that they can past the required tests. Garbage trucks, 18-wheelers, delivery trucks, you name it, they could all potentially have an 18 year old behind the wheel.
What a driver that young cannot do is cross state lines. Drivers must be at least 21 to be an interstate trucker. That is, unless a recently added provision in the six-year highway bill currently being proposed passes into law.