We love to draw lines in our modern world. We enjoy keeping order by creating clearly defined and fixed rules and regulations about what can and cannot be done. You must be 21 years old to drink (at least in America), you must be 16 years old to drive, you must be a male to drive (until recently in some countries), you must be a man to vote (sadly still the case in some countries), you know how it goes. And with children, they start battling and aiming to cross these lines from a very early age. They have to be a certain height to go on some rides, they have to be below a specific age to get free stuff and they have to be with an adult to cross to the deep end of the pool even if they argue that they can swim. In our case in Shanghai, they are most definitely better swimmers than the lifeguards, who save drowners by pulling them out using a long pole thingie because they are scared of the water!
This is the argument I have to listen to every time I am at the pool from my daughter and her friends!