Possibly! We'd both have to spell out a lot of details to have that argument. I'm not the one to have it either, because I don't know how plausible it is that we can really solve aging anytime soon.

But I'd say it's not super-relevant to the argument at hand, since the person who we're discussing doesn't say "these environmental problems are less important than aging" but just denies the specific harms are happening on the scale they are.

Edit: crap, left out a not. "It's not super-relevant."

Or more generally, people could argue about costs of different ways of mitigating deaths because maybe some non-environment-related issues are "cheaper" or "better" in some way to mitigate. There's not just the question of what causes the most deaths, but also the question of what could be done about it and how.

And people do have lots of ideas and arguments about those questions, like effective altruists' ideas about cheap medical interventions.