I'd say the #1 reason for not letting people under 21 drive interstate is because of their horrendous safety statistics and the tendency for younger people to be more aggressive, less patient, and greater risk takers. In trucking, it's all about liability and trust. Trucking companies need people with a proven track record of smart decision making which is why they tend to rule out anyone who has ever had a felony, DUI, or severe driving infractions. They even look at your credit score and social networking pages to get all the information they can about you. Unfortunately, someone who is 18 has never been anywhere except high school and mommy's house – they usually have no track record of decision making at that point in their lives to show they can be trusted. At least at 21 you've been out in the world for a few years (hopefully) and have shown you can be trusted (or not) as an adult making your own decisions.

Naturally the military loves 18-21 year olds because most people at that age are a bundle of fearless adrenaline junkies – idealistic, looking to make their mark in the world, and easier to mold. That's not a complaint against the military at all – it's just a simple matter of finding the right person in the right circumstances for the job.