A ton of pilots were employed by the so-called "commuter" airlines when they were operated under Part 135. Part 135 has no age limit for pilots. But, after the Roselawn Illinois crash of an ATR-42, virtually all scheduled Part 135 operations were forced to comply with Part 121, including the age 60 rule. Many pilots were forced to retire because of age. I remember a press conference wherein the DOT secretary stood in front of the wreckage and declared that he was going to require a higher level of safety by making all commuter aircraft operate under the "big airline" rules. The strange part is that the ATR-42 that crashed was being operated under Part 121 at the time of the crash. Ain't politics wonderful?