I know you're attempting to explain the usage of the term, not defend it, but for those who are, I feel like that's a slippery slope and that in other contexts reappropriating an otherwise neutral signifier into what is functionally a slur would immediately be identified as unethical. It's like people using "gay" to mean "stupid" or "uncool." And, for anyone outraged at the comparison, no, sexual orientation and a person's age aren't the same thing. But they're both facets of people that are beyond their individual control and ageism is a genuine form of discrimination.