It is not in the least bit clear to me that the meaning of "old" can be assumed to stay constant as lifespan increases. It might turn out, for example, that scientists who live five hundred years on average don't become old in the sense you mean until their fourth century or so.
Of course, nothing like that will be true for statistical lifespan increases primarily driven by lowering child mortality, so I'm not sure what data we could gather prior to actually having life-extension capabilities.
It's the only absolutely reliable way of getting rid of bad leaders.
This might not be a good enough reason to oppose longevity tech, but I don't think it's easily disposed of.