"Imagine if the state said that you may not quit your job on grounds that you dislike your boss's age, race, religion, or sex. If that is your reason, you must stay working there. We would all recognize that this is a case of involuntary servitude. It is an attack on freedom. So why do we not see that it is the same with the employer?
Under freedom, if an employer decides, for no good reason, that employees should not be older than 40, that is his judgment. If it is a bad decision, the competition will gain an edge by hiring the people who have been passed over.
A final point about the employee. Would you want to work for a company that doesn't really want you there, that is only maintaining your job for fear of the bureaucrat? That is not a prescription for a happy life. The happy life comes through permitting maximum freedom to associate and choose — a freedom that applies to everyone and under all circumstances, without exception."
Should Age Discrimination Be Legal? - Llewellyn H. Rockwell, Jr. - Mises Institute