In the pilot, Mindy says she's 21, which would make her the class of 1975. In this episode, Mindy attends her 10 year high school reunion for the class of 1971.
Mindy's age is stated repeatedly as 21 in the first Season, and celebrates her 22nd birthday during that first Season, becoming 23 mid way through Season 2. However, by Season she's attending her High School Reunion, aging her up to 28.