Severance buys a release of claims. For that release to hold, it has to be supported by consideration the employee was not already entitled to, and it has to be knowing and voluntary.
Age discrimination claims add specific requirements under the OWBPA, including a written advisory to consult an attorney, a consideration period, and a revocation window afterwards. Miss them and the release does not cover the claim you most wanted covered.
Certain claims cannot be released at all. An agreement that purports to bar an employee from filing a charge with the EEOC creates a new problem rather than solving the old one.

