[derefr]

> You're in the first year before your first birthday, being zero (whole) years old.

I would argue that your "first birthday" is, in fact, the day you are born—your birth day. The thing that happens for the first time a year later, is the first anniversary of your birth day.

[wlesieutre]

And I'd say that the word "birthday" is defined to mean "anniversary of the day you were born"