The average age of cars and light trucks in the U.S. has increased to 10.8 years, an all-time high, according to a leading U.S. auto market research firm on Tuesday.

Passenger cars showed a modest increase in age since 2010, from 11 years to 11.1 years at the end of June 2011, while light trucks showed an increase from 10.1 years to 10.4 years, according to the survey by Polk.

The medium age of a vehicle has increased steadily over the past 16 years, Polk said, attributing the increase to factors including improved vehicle reliability and the economic downturn that has prompted more Americans to hold onto their old vehicles rather than buying a new car.

"The increasing age of the vehicle fleet, together with the increasing length of ownership, offers significant business growth opportunity for the automotive aftermarket," said Mark Seng, global aftermarket practice leader at Polk.

"Dealer service departments and independent repair facilities, as well as aftermarket parts suppliers, will see increased business opportunities with customers in need of vehicle service," he said.

In July 2011, the total vehicles in operation in the U.S. was over 240.5 million, an increase of 500,000 units over July in the previous year, according to the Polk report.