Proverbs 22:6 says, "Train a child in the way he should go, and when he is old he will not turn from it."
Let's analyze this one sentence. The Bible says that when he is old he will not turn away from it. Turn away from what? Turn away from what the child is trained to do - the lifetime habit that the child has acquired, good or bad. When you train a child since the child is a kid in the way that he should go, day in and day out, it becomes a part of him - his habit. And once a habit becomes a part of an individual, it will be extremely hard to change a habit, whether it is a good habit or a bad habit. And don't forget, although a bad habit can be temporarily suppressed, such bad habit in "remission" can relapse. A bad habit that is suppressed can return under the right condition, just like a mental illness. Read here.
The apostle Paul describes how hard it is for him to change a bad habit,
14 We know that the law is spiritual; but I am unspiritual, sold as a slave to sin. 15 I do not understand what I do. For what I want to do I do not do, but what I hate I do. 16 And if I do what I do not want to do, I agree that the law is good. 17 As it is, it is no longer I myself who do it, but it is sin living in me. 18 For I know that good itself does not dwell in me, that is, in my sinful nature. For I have the desire to do what is good, but I cannot carry it out. 19 For I do not do the good I want to do, but the evil I do not want to do—this I keep on doing. 20 Now if I do what I do not want to do, it is no longer I who do it, but it is sin living in me that does it. Romans 7:14-20
Paul talks about sin that is living in him. What is "sin" that lives in you and me? Can you figure that one out, a sin that lives in us? The habit of stealing is a sin. The habit of lying is a sin. The habit of hacking is a sin. The habit of cheating is a sin. Disobedience to God's law is a sin. The habit of dishonoring and abandoning our poor parents is a sin. All these samples of bad habits live in us. They live in us because we own them - part of our character.
God will hold us all accountable for our bad habit - the sin that lives in us.