Its not just numbers, but percentages: Whereas five years ago a full 17.4 percent of used cars were sold for use by teens, this year it's less than 11 percent.

Of course, the economy is only one explanation. There is also a trend, it seems, of younger drivers waiting to get their licenses. Holding off until 18, rather than getting a license two years earlier, allows younger drivers to avoid the many legal restrictions placed on them detailing what they can and can't do behind the wheel, such as curfews and the inability to drive with other young passengers.

One silver lining, at least for the Detroit Big Three: Of the used cars being sold to teens, a vast majority are from America's major manufacturers.