I’m thinking something like 25.
It’s not until age 25 that the part of the brain responsible for complex judgments and decisions comes fully online. If you’re trained only in non-lethal combat, and you find yourself compelled to self-defense, then we as a society are willing to accept a certain amount of bad judgment on your part. Maybe it wasn’t really self-defense, but the damage you’ve caused is fixable. On the other hand, if you are trained to kill, then you are a menace to us all until you have a very solid and very nuanced understanding of what constitutes self-defense.
Note that all of the above assumes a fairly safe environment where being attacked with a knife is extremely unusual. If you as the parent determine that any killing the child does must necessarily be in self-defense, because murder awaits him behind every corner, well, I pray that you can find some way to get yourself and your loved ones out of that war zone.
Until then, I say begin his/her training as soon as he’s able to grasp any object. Surely he doesn’t need a lethal weapon in his hand in order to learn how to wield one. And it’s not all about the knife, either. He’ll need to know how to get out of the way of the other guy’s knife. He won’t need a lethal weapon in his hand for that training either. At what age put a lethal weapon in his hand? At the same age you let him start going out the front door, if you live in a place like that.
Peace and luck