Many people refer to sex as what someone is biologically and gender as what society considers them (edit: just FYI I'm not an expert and this is probably an oversimplification). While sex is more constant there isn't any reason gender has to be (and there are practical reasons why it shouldn't be but that's out of scope of this comment). This means that while biologically someone might have two XX chromosomes and be 60 years old it's not necessary that society considers them an old woman.

In what sense is the time since birth subjective?

Excepting general relativity time dilation Interstellar situations, that is.

That's simply wrong. Age has a definition. You cannot 'feel' yourself into a different age. Sex != gender.

> That's simply wrong.

> 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.

Quite the stretch. Ok, I give up: if it is demonstrable that a given person has traveled near the speed of light and thus aged significantly differently than the rest of us, that person should be allowed to legally change their age.

That's happened zero times thus far, so I stand by my original argument.

Physics is quite a stretch and yet denying biology isn’t... This is an anti-science, ideological stance.

>Physics is quite a stretch and yet denying biology isn’t... Who's talking about biology? Go look up the definition of "gender". We can debate the specifics and current state of affairs, but you can't argue with the definition.

The definition of gender, stated multiple times in this thread as if it were a hard truth (i.e. a social construct), is purely ideological.