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If you believe that you are being retaliated against because of your FMLA leave, then you can file a complaint with the Department of Labor, which is the federal agency that investigates and prosecutes such matters. If you believe that you are being treated poorly because of how old you are, you can file a complaint with the EEOC for age discrimination. The EEOC will conduct an investigation into your employer's practices, as well.
Here is a link to where you would file your DOL complaint. Here is the link to where you would file your EEOC complaint.
It should be noted that filing either of these complaints is a form of legally protected activity. This means your employer will be legally prohibited from retaliating against you, and any adverse employment action which you can link to the filing of your complaint with give rise to an entirely separate legal claim for damages.
These would be your options under the circumstances you describe, generally speaking.
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