I think that's a big part of it. Kids don't have as much time to devote to the hours they need to log. Also, I know of several kids that don't have the money for insurance so they haven't gotten their licenses. And I even know of one teen-age boy for whom driving makes him nervous, and he just figures that for now he has enough friends who drive that it doesn't inconvenience him. The article I read actually sites all the new technology as a reason for more kids driving later -- driving isn't the end-all and be-all of socializing that it used to be. In the end, though, I think that it just means that the 16th birthday isn't quite the driving milestone that it used to be and that can only be a good thing for grade advanced kids.
