Let’s work this one out. The technical training of a boxer, like any other coordinated physical activity, is good for a child’s body at any age.
The most important question is: ‘at what age should they begin full-contact sparring and competing?
If your aim is to straighten out a little brat, then you might as well not bother reading further. The following is for responsible parents.
If you want to teach a child boxing, ingrain in them a love for the sport, pay close attention to them. They’ve picked up the stance, defensive moves and punches. They’ve become physically stronger. They’re punching the bag and the pads. They’re moving confidently, listening to their trainer, and progressing. Do they have that fire in the belly? The thrill of competition? Are they ready to get hit and not give up?
If not, then don’t agonize them. It means the time isn’t yet right. You as parents are obliged to take this into account, as is the trainer of course. The burning desire to box will be a sign that they are ready.