Wow! how lovelt catwalker. I always thought it was 21 for boys and 18 for girls.
My little brother is 6'8", but I can't remember at what age he stopped growing. My Ds is 5'5" and isn't 11 for another 4 weeks, so I'm expecting a very tall Ds too! which is lovely, but his back is very rounded because he won't stand up straight or sit straight either