According to a recent report by R. L. Polk & Co, the median age for passenger cars is 9.2 years in the US for second year in a row. It was 8.3 years in 1998. These figures have increased steadily since then. So Americans are driving older cars. If we look at what happens at the end of a car's life, 5.5 percent of cars were scrapped in 2007, compared to 9.5 percent in 1970 - so Americans are scrapping less as well.
The could be several reasons for this. Americans might not want to change their cars more often for economic reasons, because their vehicles are more reliable and have increased longevity or simply because cars don't change all that much in each model year. In addition, the question of whether if it's financially worth it to upgrade to a greener car -- regardless of how polluting an individual's current vehicle might be -- will never go away. There are no easy answers to be had.
[Source: R. L. Polk & Co]