What is the best age for children to start training?

[Nevinyrral]

Since OP trains taekwondo I would go with that for a first martial art. It might be nice idea to have kid train with parent both in class and at home.

As to exact age, I think it depends on how 'smart' he is. At the very least the child needs to understand basic commands like stand in line or sit down.

About girls in martial arts and power it will come in time. I saw 13yo fighting more ferocious than adults. Of course boys will always have power advantage.