> Although, if you show me a random person, I can easily tell you their sex (and in most cases gender).

No, you think you can easily tell their sex, because people who have successfully transitioned are the ones you can't pick out. It's confirmation bias.

So they get wrong one in ten thousand. I bet you can't guess the age of 9999/10000 people.

Maybe I don't understand your argument but it sounds like to me you would think that 3 feet long is societally determined because you need a social system to define what 3 feet is and create measuring tapes and whatever but that would make every metric socially constructed. It only works based off a different definition of what socially constructed means thqn I intended in my post so I could try to nail down a better definition or if you want you could suggest to me clearer words in the future but I'm loathe to get into a semantic argument here.

Edit: TL;DR by my understanding your definition of societally determined is so broad as to not really be very meaningful.