Age discrimination is unlawful because a statute says it is. The statute defines age discrimination as discrimination base on age over the age of 40. It does not include in its definition discrimination based on age for those under age 40. Therefore someone can openly and intentionally make a decision based on someone's age without any legal consequences, as long as the person is not over 40 years old.
Your post suggests you were treated differently because you were a man and the other employee was a woman. Gender discrimination is not so limited. Your situation might well be what we call gender-plus discrimination. In that situation, a person of one gender with a situation (that is not necessarily protected) is treated differently from a person of the other gender who has a similar situation.
The big problem you have is that the damages you suffered as a result of this conduct, if it was determined to be gender discrimination (which is still a tough case) are not very large. They were no doubt large for you, but in the greater scheme of things, it is unlikely there is enough of a damage to support the attorney fees it would take to fight the good fight.
On top of that, you risk alienating your employer by suing it over what is likely not a large sum of money.
Good luck to you.
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Not all types of discrimination are illegal. The Department of Fair Employment and Housing and the EEOC discuss the types of discrimination that are prohibited by law. Age discrimination protects older employers (40+ years old). Younger employees, under 40, are not protected by age discrimination laws. Your employer has the right to hire any employees of any age as long as they comply with the law. It may seem unfair to you that you had to wait until you were 18 to work for your employer, when your employer hired a 17 year old from a different restaurant, but your employer's conduct is not unlawful.
I am not providing a legal opinion based upon your specific case since I do not have sufficient information to form an opinion at this time and in this forum. Moreover, I do not represent any client until I have evaluated their case and we have executed a written attorney-client retainer agreement. Since all legal claims have specific filing deadlines, you should consult an experienced attorney immediately to better understand your options and your filing deadlines.