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The general rule is that employment in the state of Texas is "at will" absent an express agreement to the contrary. At will employment can be terminated or modified for any reason not amounting to discrimination on the basis of a legally protected trait (age, race, religion, gender, etc.) or retaliation for engaging in certain forms of legally protected conduct (filing a wage claim, taking FMLA leave, etc.). It doesn't matter whether the basis for termination/modification is fair, reasonable or even true.
Therefore, in order to have any legal claim against your employer, you would need to prove that they did this to you because of how old you are. The fact you were replaced with someone younger is suggestive of that, but since the burden of proof ultimately will fall on your shoulders as a plaintiff, you will need something more. Have derogatory comments been made about your age? Is the replacement employee drastically under-qualified? Did they ever indicate an intent to replace you with someone "younger" or "fresher"?
If you believe you can make a case that this is happening because of your age, or perhaps because of your disability (also a legally protected trait), then your first step is to file a complaint with the TWC Civil Rights Division. See here for more information.
The Civil Rights Division will investigate the matter and if they find cause to believe there is illegal discrimination, will attempt to mediate a resolution with your employer. Perhaps your employer will revert you back to your position or modify your new position through this process. But if they don't, the Civil Rights Division will issue you a "right to sue" letter, and at that point you could file a lawsuit in civil court.
I hope that you find this information helpful. Please do not hesitate to let me know if you have any questions or concerns regarding the above and I will be more than happy to assist you further.
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