Perhaps I wasn't clear.

The ruling wasn't that Part 91 pilots *had* to abide by the 121 mandatory retirement age...simply that if a Part 91 flight department used the 121 mandatory retirement age as policy it did not constitute age discrimination.

Federal Court: Mandatory Retirement Age for Pilots Is Not Age Discrimination | Age 60 | Personnel Considerations | NBAA - National Business Aviation Association

In short - there is no regulatory mandatory retirement age for Part 91 operators (in part because private carriage is not common carriage) but operators are legally entitled to set an age limit as a BFOQ.