As a parent the only thing that should matter to you is developing your player into the best player he/she can be. Not winning or losing. Development.
This is sort of scary turf. If I would have known this when my son was this age, I'm not sure if I would have had the same approach with him. Which may have helped or hurt. Who knows? But, looking back in retrospect I'm just glad that I worked hard at researching and learning the game I love. I have a small baseball library and video collection. It was fun learning and fun teaching it to my son. Let me make it clear that he has passed me by long ago in his professional career. I'm talking about the kid and playing with him in the backyard and teaching him the game as best I could. Not the professional today.
Both my son and I have worked with kids and at about 8 or 9 many of them are ready for some basic instruction. My experience with kids between 13 and 16 has been poor. And, what's sad is those older kids are really wanting to learn and become better players. Unfortunately they have performed bad mechanics over and over. They have etched the wrong movements into their brains, or into their muscle memory. And, they just can't get these bad habits out of their systems.
A book that made me feel that I was right about this fact is, "Why Michael (Jordan) Couldn't Hit, and Other Tales of the Neurology of Sports", by Dr. Harold Klawans. This is an interesting read and I will be mentioning it again latter. His knowledge of the development of the brain and nervous system tells us that by 12 years of age, players with bad mechanics and kids who haven't played are unlikely to develop proper mechanics. Or, they won't be able to develop the mechanics that will make them successful as baseball players.
I've had parents ask me since my son was 6 years old how he had become a good player. At the age of six I just told them the truth. When he wants me to play with him in the back yard I do it ever chance I have. As he got older I'd just tell them we have a batting cage and he loves to take grounders. Basically he liked to hit, throw, and field. I never felt comfortable talking like I was some baseball guru because I wasn't and I'm not. But, I found what worked for my son and numerous other young players too.
So the window in time to develop proper mechanics is from 8 to 11 years old. Maybe a little younger depending on the kid.
The biggest hurdle you are going to face is instructing and keeping it fun. Remember when you are playing with your youngster in the backyard he/she doesn't need to understand proper mechanics, they just need to do them.
Next: Where and when to start teaching your son proper mechanics.