During the first two years of Covid there was an exemption from the hours limits for "essential" goods. There was talk at the same time about lowering the age to 18, but thankfully nothing came of it. States can license truck drivers under 21 for intrastate driving; I don't know how many there are.

There's a lot of class A intrastate drivers but not as many tractor trailers. Think box trucks, dump trucks etc. I'm not sure, given how boxes are loaded, how many people got the hours extended. I know my company didn't even bother with it