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I'm sorry to hear about your situation. While they can't discriminate against you solely based upon your age, that doesn't mean that they cannot hire someone younger if they lay you off.
In your case, you are covered by Title VII and the ADEA (Age Discrimination in Employment Act), meaning that you could, potentially, have a case and complaint against your employer. That being said, age discrimination cases are difficult to win. In order to show a prima facie case for age discrimination in reductions in force, you are going to have to show: (i) your were in a protected age class (check), (ii) you were discharged (check), (iii) at the time of your discharge, you were performing your job at a level that met your employer's legitimate expectations (likely), and (iv) persons of your qualifications were retained in the same position or there was some other evidence that the employer did not treat age neutrally in deciding to dismiss plaintiff (more difficult).
In ADEA ("age discrimination") cases, the plaintiff must present evidence demonstrating a "reasonable probability" and not merely a "possibility" of age discrimination. First, a plaintiff can offer direct and circumstantial evidence that he would not have been discharged but for his age; the evidence must be of sufficient probative force to support an inference of discrimination. Second, a plaintiff may use the McDonnell Douglas scheme of shifting burdens applied in Title VII cases. Under this scheme, once the plaintiff has established a prima facie case of age discrimination, the burden shifts to the employer to articulate a legitimate nondiscriminatory reason for its employment decision. If the employer meets this burden, the plaintiff must then show that the employer's proffered reason was mere pretext and that age was the more likely reason for his dismissal.
The Supreme Court has expressly held that an employer does not violate the ADEA "by acting on the basis of a factor, such as an employee's pension status or seniority [or salary], that is empirically correlated with age." Even though age is often related to factors such as salary, it is "analytically distinct" from them. For this reason, "an employer can take account of one while ignoring the other." Moreover, "[w]hen the employer's decision is wholly motivated by factors other than age, ... [e]ven if the motivating factor is correlated with age," that decision is not contrary to the ADEA. Accordingly, even if the company reduced the salary to reduce its costs (or even fired The employee simply to reduce its salary costs) this is not evidence of age discrimination.
Unless you can establish that their actions were only because of your age, and not some other factor (such as being on medical leave) then I'm afraid to say that you would not have a case. You'd have to prove that you were laid off because of your age, and unfortunately the facts that you have presented make it seem that your age is only incidental to the matter rather than a motivating factor.
I know this is probably not what you wanted to hear, but it is the law. I hope that clears things up anyway. If you have any other questions, please let me know. If not, and you have not yet, please rate my answer AND press the "submit" button, if applicable. Please note that I don't get any credit for the time and effort that I spent on this answer unless and until you rate it positively (3 or more stars). Look for the stars on your screen (★★★★★). Thank you, ***** ***** luck to you!