The reality is that it depends on what do you want to achieve.
I personally started my kids training in martial arts when they were newborns. Of course this doesn’t mean that they were in a school as babies learning how to use baby bottles and pacifiers as weapons.
Their training as babies involved muscle development and muscle memory and the way I taught them was using physical manipulation.
The key here was that I wanted them to grow healthy and worked with different lesson plans to stimulate different areas of the brain as they were growing up.
The results for me speak by themselves. Each one of my kids is different but both of them can accomplish incredible mental and physical feats.
However when dealing with private schools each one is different and follows different protocols.
For me and my instructors we prefer to start the kids at five years old and occasionally have waived the rules for some four years old. So my recommendations are to you are: 1. Decide upon your goals; 2. Interview the potential instructors; 3. Visit the schools and have your kid try the classes; 4. Ask other students and parent; 5.Read all papers and contracts carefully;
Occasionally I have recommended parents of younger kids to start gymnastics or soccer prior to engaging in martial arts.
Whatever you decide remember that not all kids and schools are the same so try to match goals, kids and schools before engaging into any contract.