It sounds like there may be a potential age discrimination case. But they are very hard to prove, and litigation is tough. I think it comes down to whether the severance payment is significant to make a difference for you.
A thorough evaluation of your potential claims would require you to consult with a local employment attorney. There are a variety of factors that would need to be considered to determine whether you have an age discrimination claim.
In any event, you should have an attorney review the severance agreement before you sign it. You should have been given at least 21 days to review the document and then there is a 7 day revocation period where you can change your mind and revoke the agreement. I would use that 21 day period to have an attorney review the document with you and to discuss your potential age discrimination claim. Good luck.