(a) Continue doing the best job you can, so that any argument that the reason for your termination is your performance becomes week and unconvincing. Poor performance is one of the employers' favorite excuses for terminating an older worker, so the better your performance is, the harder it will be for the employer to defend the case.
(b) Keep track of all the older people who are being "laid off" or replaced with younger ones. Find out ages of those who are fired and hired. This will help you determine whether there is a clear pattern of discriminating against older workers. This can be very useful in proving your age discrimination case in the future.
(c) Pay attention to any of management's statements, e-mails or postings on social media or anywhere else that in any way suggest that they prefer to hire and keep younger employees over older ones. Ask your co-workers if they have heard anything about the employer's attempts to get rid of older workers or about the company's strong preference for hiring younger employees. This type of evidence, even if it's just one e-mail or one statement, can make a difference between having no case and having a case, and between having a weak case v having a stronger case.