> Age has a definition.

Relativity explains that this definition is subjective, experimentally.

Astronauts experience time differentials and it will only get more obvious as we go farther. Again, the concept of granularity (how much of what matters from what perspective) applies in both cases - age and gender.

This isn't a justification. Resolve this philosophically however you like with one and I observe that it should be true in the other case.