Children are stupid. How does putting them together by age improve the outcome of education? As an adult, do you normally interact only with people your same age? 

 “ putting them together by age improve the outcome of education” 

no, that’s not what I said at all, neither my wife thinks that

“ do you normally interact only with people your same age?”

The key word there is “only”.  No I don’t only interact with people the same age, but mostly yeah. That’s normal.  

 Put children together of the same age, the girls will all compete to be the same, the boys will all compete for who is the most adult. If half the girls are raised by whores, all the girls will try to dress like whores. If one boy is neglected by his parents and he has to grow up fast, and is a rebellious troublemaker, all the boys will compete to be like him. The school system is designed to turn out broken people, by intention.