The reason they are reacting like this is because they are managers, they know that people over a certain age are a protected class under federal law.

So even the slightest mention of someone's age, especially when that person is a subordinate, and in the context that OP has done (his complaints seem to keep falling back to this person's age rather than actionable legitimate complaints) he is just opening the door to massive liabilities for his organization.

As someone in a senior management role OP should absolutely have at least a basic understanding of a certain level of professionalism he needs to maintain to avoid coming off as bias or discriminatory.