um.... it doesn't.
free admission on your birthday was in 2009.
you're 5yrs too late for that.
when that promo was going on anyone could get free admission on their birthday.
you could show a birth certificate if you did not have another form of ID such as a license or passport.
the only kids that get in for free are children ages 2 and under and its all the time not just on their birthday and you don't need to show proof for that.