I believe the criterium class for 50-59 is Masters C and 60+ is Masters D. OFTEN, the organizers will combine the two classes and may even combine them with the immediately younger class. (There aren't many things less fun than racing a criterium as a 57-year old up against 45-year olds.) Sadly, even with as many races as they hold over the long weekend, Tulsa Tough does not include Masters C or D classes.
But the hint has already been put out there . . . Masters bicycle racing is reputed to be the most chemically enhanced racing in America. (It is rather remarkable how much faster some cyclists become after they turn 45.)