The trend can be attributed to consumers who don't want to spend lots of cash on new vehicles, and to improved quality that keeps cars on the road longer, Polk vice president Mark Seng told The Detroit News.
Last year, the average passenger car age was 11.2 years. In 2007 it was 10, and in 2002 it was 9.6.
The figures are based on registrations for cars and light trucks.
While the US Public Interest Research Group has produced evidence that the steady rise of cars on the road in the past six decades is over, Polk expects a rebound in new car registrations (though it says average age will continue to rise). It estimates the total number of cars in the country will hit 260 million by 2018.
Based on July sales figures, the "Detroit Big Three" all missed expectations, but major automakers posted significant gains, in line with an upward trend in American