Mark Zuckerberg, the CEO of Facebook is only 24 years old. Unfortunately, all of us at various points, may have to get used to reporting to someone younger than us. Even though the person is youger than you, do they have experience that you do not have (for example: management experience)?
The answer to your question is "maybe." Your question requires a lot more information to assess.
First, how many employees does your employer have? If you are in California, the Fair Employment and Housing Act covers employers who have five or more employees. If you are not in California and your state does not have its own age discrimination laws, the federal Age Discrimination in Employment Act applies to employers with 20 or more employees.
Second, did you suffer any adverse job consequences as a result of the younger person being hired as your supervisor? Has anyone made age-related comments to you, such as statements that you are too old to fit in the company culture, or that you might need to retire soon? If you were not demoted or denied a promotion (for example), then was there any other change in the compensation, terms, conditions, or privileges of your employment? Did they change your job duties? Take certain duties away? Lower your compensation? Make it more difficult for you to get a promotion? Is there anything else that the company did to make your job less desirable or remove responsibilities that you had prior to the younger person being hired?
Third, how much younger than you is the person who was hired? Under the law, there needs to be a "substantial difference" between your age and the age of the younger employee in order for the age discrimination laws to be potentially implicated. The term "substantial difference" has been interpreted differently in various jurisdictions, ranging from a five-year difference to a difference of more than five years.
Fourth, what did the company tell you when they put the younger person in the position as your supervisor? Did they say anything to suggest that the person was selected because he or she is younger than you are?
Fifth, how do the younger employee's qualifications compare to your own? Is the person equally qualified, less qualified, or more qualified?
There are additional factors to consider as well, but you should contact an employment lawyer to discuss whether or not your employer's actions violate antidiscrimination laws. Good luck.