How many age discrimination cases are there in the U.S.?

At least 327,143.

From 1992 up to the end of 2009, 327,143 cases were filed under the United States Age Discrimination in Employment Act or ADEA. This does not include the number of cases filed since the law was codified in 1967 up until 1991. This data is from the ADEA Enforcement and Litigation Statistics of the  U.S. Equal Employment Opportunity Commission. In 2009, 22,778 cases were received by U.S. courts, with about 90% of these resolved.