Is it discrimination for a night club/bar/restaurant to charge a cover for people ages 21-25 only?

Age discrimination is only illegal as to people over 40.  I'm guessing the club has found that the behavior of young adults is costing it extra money in security and insurance, as well as expenses dealing with licensing authorities and the city, and that more mature patrons do not engage in that kind of expensive behavior.  Just as car rental companies can charge more or refuse to rent to younger drivers based on their driving habits as a group, the club can charge what it feels is appropriate and necessary to cover it's extra expenses and risks from accomodating people in that age group.