You need to have a sit down consultation with an employment attorney. Although you think that your were terminated either based on your age (violation of Title VII possibly) or because you took FMLA leave, every case is based on what you can prove and what you can not prove. You will have the burden of proving by a preponderance of the evidence that this was a retaliatory termination in a state in which you are considered to be an "at will" employee. So, it is an uphill battle. Best wishes to you.
The information provided in this answer does not create an attorney-client relationship. If you are interested in his legal services, feel free to call Chris at (303) 409-7635 at his law office in the Denver Tech Center. All initial consultations are free of charge.
The reason we have federal laws to protect employees is because unfortunately, employers still occasionally do things that are illegal. Every lawsuit is an "uphill battle," but that doesn't mean that employees have no protection under the law. If you think something was wrong with the way you were terminated, don't let the previous answer discourage you from speaking with an employment attorney who routinely represents employees in these types of cases.
The reason you need to sit down with someone is that employment law can be very complicated. The facts you have included are important but there are sure to be many additional facts that shape your (potential) case. There are also likely to be some different options for pursuing your (potential) case. If you want to take legal action, you will need to meet some deadlines under applicable laws, so don't hesitate to sit down with an attorney.
Notice: My answer to your question is general information. It does not create an attorney-client relationship and is not governed by confidentiality rules. This general information is not a substitute for sitting down and discussing your circumstances with an attorney.