For trucks, the median age was 6.6 years because of the record 9.8 million new trucks registered in 2004.
Passenger cars comprised 59 percent of all vehicles on U.S. roads last year and trucks 41 percent.
"Since vehicles are lasting longer, and new light truck registrations around 9 million, we will see a noticeable change in the proportion of cars versus light trucks in the near future," said Marty Miller, product manager of the automotive information service.
The percentage of total U.S. vehicles scrapped in 2004 was 5.3 percent, the lowest in two years. The passenger car scrappage rate was 4.8 percent, the lowest in 56 years. The overall rate of trucks heading to the junkyard fell to 6 percent from 7.7 percent in 2003.