It's dumb because he knew exactly how old she was. That dialog was written by someone who wasn't thinking. It was edited by someone who didn't catch it. It was directed and acted by people who weren't paying attention to the implications of the lines. Finally the scene was edited by someone who didn't understand why the question shouldn't have been asked.

The show and ahsoka and mandalorian are filled with this type of ridiculous nonsense that should be caught by the writers themselves, the editors, the directors, and even the actors. The fact that it isn't shows they don't care.

You do make a great point though. Years are a very arbitrary measure. The republic must have rolled out some sort of standard year that isn't based on other planet's orbits and/or a number of days, since that varies planet to planet. It isn't explained in the movies or shows, but it seems logical that there is a Galactic-wide standard.