A couple weeks ago, I realized that I didn't know how many weeks old my daughter was. Moreover, I started to think of her age in months rather than weeks. Eventually, I'll start thinking of her age in fractions of years...then years. I will probably think of her grade as her age for a little while. Then it will go back to years.
Right now, my daughter is still X inches long. At some point, that will become "tall" instead of "long." Then feet will be added to the equation.
Right now, I think of my daughter as weighing X pounds Y ounces. Pretty soon, ounces will no longer be a consideration.
I never guessed how much fatherhood would make me aware of definitional terms.