Bringing a birth certificate does not hurt but currently they play it by ear.
The most common test is asking the child himself how old he is.
if they asked my grandson how old he is-- he will tell them 5. He will be 2.7 and VERY big. His sister is 9 and 5'4' and wears a 9 show. THe parents are 6'3 and 5'9
I really do not think they will ask how old the child is.
I am not taking a birth certificate as i take enough stuff to get me through the parks each day