reasoning? I mean, I want to hear legitimate reasoning as to why or why not?
I have copied below the answer I gave to a father asking about coaching his 6-year-old son for the 400m. the specific reasons why you should not do this are exactly the same as they were for him.
I'm the old-fashioned type who thinks that words have meanings.
play means, participate in a sporting event, a game or race of some kind. kids as young as six can play ball games, have running and swimming races, and they can compete against kids their own age. this is fine.
train means to practice specific skills and develop specific physical characteristics in order to be able to play sport better. it literally means to induce physiological changes that make the person better suited to some specific activity.
the first law of training is the law of specificity. it says that training induces physiological changes in the body that are specific to the training you do. we read this both ways. it means, you become the training you do, and it means that you choose the training that makes you what you want to become.
training, is then not a process but a journey. it takes you from where you are (physically and physiologically) to where you want to be. since your boy is just 6-years-old, you have no idea where you are (physically and physiologically speaking), and you will not know until he is much closer to puberty.
I have been an athletics coach for thirty years, and I have coached pretty much exclusively 14-16 year-old boys for twenty of those years, and it is my considered opinion that no child should commence training until they have been through puberty. the boys I coach do not do event-specific work until their body is ready to commence adjusting to the rigours of their chosen event. it varies from boy to boy, but generally, by the time they are 15, they are ready to START. if you start a boy training at 6-years old, you are essentially trying to mould him before you even know what material you have to work with.
play games, by all means. participate in competitive sports against kids his own age, by all means. just don't do any training for this. make it play, an adventure, a generally fun thing he can do with his dad. but avoid coaching or training or any kind of deliberate attempt to make him better at it until he is a lot older. another eight to ten years older.
cheers.