Q. "How do we discipline an employee who is no longer meeting expectations but also part of a protected class?"
Answer:
In general, you shouldn't let an employee's protected class status deter you from holding them accountable to expectations. Everyone belongs to protected classes and being in one simply means that person can't be treated differently because of their inclusion in that class.
Now if your organization has a history of bias or holding members of a certain protected class accountable for poor performance when others aren't, then disciplining this employee would raise your risk of a discrimination claim. Assuming that's not the case, there's no reason to worry.
When you do discuss performance with this employee, don't reference their protected class (i.e., attributing their drop in performance to getting older). Instead, focus on documented, specific examples of how their performance isn't meeting your expectations, what steps have been taken to address them, and what needs to change going forward.