See, AGE is something that we can actually measure and know FOR CERTAIN how old someone is, based on their birth certificate.
Something as nebulous as "sex" cannot be ever measured. We can't know whether a person has two X chromosomes or an X and Y chromosome, or even if they have a twig and berries or not!
So, OF COURSE, AGE must be set in stone. It's not like it is a state of mind, like sex is.