Any age below 15 is too young for a regular lesson. If started at a slow pace and with a training time based on the individual child's attention span then about 5 is ok. The thing I don't believe in is putting children in large groups and with a teaching structure identical to what teenagers and adults have. I base my opinion on history and my experience. About 14-15 is the age when most of the older okinawans started and it goes back to Matsumura Sokon perhaps further. Also kids under 15 are still learning to control their own bodies and too young for serious training. It is better to teach them general movements in small groups.