Some say Pro. 22:6 only indicates a principle. I don't know what that would mean. Some, are more confident, and say it is a general promise. I go a step further and say Pro. 22:6 is a fact.
What do I mean? Well, children are being taught many things. Children go the way they are trained to go. It's true in many ways. However, only a few, even only a few Christians, actually teach their children to go the way of God. Many will defend parents of children who have gone astray and say they seemed to have tried. But what I notice is that parents often allow too much of the world into their child's life. The music is carnal and there's little to no control or input from the parents. Video games and television are more of an influence than is the church. It's a huge mistake. The peers are pulling their children away and yet the parents feel it isn't their place to decide who their children make friends with.
Instead of dismissing the reality of what is said in Proverbs 22:6, we should first examine how many parents actually "train up their children in the way to go".