There may be other explanations, both lawful and not, but employment discrimination can be proven by inference. The employer will always deny discriminating, but it may turn out that's the case. once the decisionmakers have declared themselves as to their explanations and all the other evidence is in.
This is not formal legal advice.
If you were an employee at will (no union, no employment contract, not a government employee), then you can be fired for any reason, no reason, or a bad reason. The exceptions to the employment at will doctrine are an employer cannot legally terminate your employment because of discrimination based on a protected category such as age, race, sex, etc.. or another unlawful motive.
The fact that you are 65 means you are protected by the ADEA, but that doesn't necessarily mean you terminated BECAUSE of your age. These cases depend on a number of factors, which a local employment attorney would need to explore before determining if there is a viable case. Try the Find A Lawyer tab on Avvo or the National Employment Lawyers Association's website. Good luck.