That's not true. In most applications (oil fields are exempt) there are strict rules on time driving and per shift. Still have to be 21 to have a CDL. They are getting younger because the boomers are retiring

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.