I was trying to get it setup so that you could see a users age, but not what day of the year they were born.
The greyed out arrow in the individual user options indicates this isn't possible
Then I realised of course that it's kind of pointless showing someones age, and not the day they were born as you'd be able to figure out the day as soon as their age changes.
So I went back to the default user registration options and the options there are laid out slightly differently, which doesn't make it as clear.
It lets you think you can do what I originally wanted, just showing the year of birth without the day and month, when really this should be an option.
It might be worth editing the default registration values so it's got the same conditional as when a user edits it themselves.