Under the Age Discrimination in Employment Act, it is unlawful to discriminate against a person because of his or her age. This law applies to those 40 years of age or older, and it applies to all aspects of employment: firing, promotion, hiring, compensation, benefits, training, and job assignments. The law applies to employers with at least 20 employees.
To prove age discrimination you would need strong direct evidence that your employer engaged in discriminatory activity against you. Examples of such evidence could include statements, email, or correspondence from the employer admitting or strongly implying that age was a factor in the decision to promote or hire a younger employee.
In some cases, you may be able to prove age discrimination by demonstrating that your employer engaged in a pattern of discriminatory behavior. For example, if you do not have strong direct evidence of age discrimination, you might still be able to establish it by showing that a younger person receives a promotion over an older person virtually every time. You would need to talk to an employment attorney to determine what might be necessary in your particular situation.