Talking of swimmers, we were at a friends 40th birthday party with our kids on Saturday and a lot of the kids there were the same age as my ten year old son as they all know each other from the same NCT pre-natal class (well post-birth, but you know what I mean). Now my ten year old is no slouch being the tallest (and widest) in his class, but a girl born at the same time as him whom he used to play with was as tall as my wife (5' 7") and is the fastest breast stroke swimmer in the country for her age. She is still just ten and has size 10 feet. And she is still growing. She has to race against 14-15 year olds as there is no real competition in her age range. She is also a black belt, first dan, in Karate which will come in handy later.
I suspect eldest son will still want to play with her in a few years time, but he won't be tall enough.
