Why?

I think for the foreseeable future, we’d likely still track age via Earth years for the sake of avoiding this kind of thing

This is actually how it’s handled in the science fiction universes. Star Wars for example uses Corusant time as the standard across the galaxy.

It’s not about your age difference. It’s just hitting 30. My wife is 5 years younger than me and I couldn’t tell you how old either of us are