The Bible teaches that the law is the instructor by which we know we are sinners. It teaches that those who know no law still have the sense of distinguishing some things are right and somethings are wrong... as in a man understands his knowledge of possessions that what he owns or makes is his until such time as he trades it with another..... in which case of ownership.... if he takes something which doesn't belong to him or someone takes from him a possession without his agreement.... the law of injustice is already written within his heart to which he is accountable to obeying and will serve to judge him if it brings him not to repentance if he has stolen or it will drive him with desire for retribution for a person who violates his ownership. Hence, experience in life brings a degree of understanding some things are right and some things are wrong without the formality of being taught.... and that becomes a law to the ignorant but 'natural man' ..... and that 'law' demands a penalty or making 'right' to return parties which offend to rightful relationship again. Jesus said he came not to do away with the law but to fullfill it. Law requires obediance or penalties for disobediance: There is no 'middle ground'. Breaking one point of the law makes one 'lawless' no matter how well the rest of the law was kept.....in God's sight. Jesus paid all the penalty which God demanded for the remission of our sin by his substitutionary death on the cross..... the wages of sin is death, without the shedding of blood there is no remission of sin; The human priesthood carried out repetitive animal sacrifices for the sins of the people... but the blood of animals was imperfect and a foreshadow of what God had planned to become the Savior for his people and for all humankind who would receive him.
The child sins because he is a sinner: He is selfish, rebellious, filled with his own desires and demands for attention, and covetous..... not mindful if another child he hits is hurt or that his bite is wrong but relishes in the response of the victim: However, young children may not understand the law...... even for a while though they are taught 'rules' to go by. Their way of thinking may not yet be mature to distinguish the difference between deliberately lying vs telling you a story the way they fantasized it to be or wished it to be. Thus, in their lack of understanding, an innocence is presumed upon them, not because they were in perfect obediance, but that they neither had understanding of sin nor its consequences......knowledge that they trespassed and understanding that their trespass was unacceptible. this is 'accountability.... and the question then becomes 'accountable to whom?' Speaking to this end, after healing the blind man, blind from birth, Jesus made a rebuke to the religious leaders of his day.... that if they were blind then they would be without sin.... but since they say that they 'see' .......
James, the brother of Jesus, speaks beautifully, in his epistle, how we are like children.... but if we sin..... we confess and repent. Jesus seemed to be delivering a message by example beyond that of the master being servant of all.....when he came to Peter to wash his feet... and Peter at first refused... but the Lord rebuked.... and Peter repented saying not his feet only but wash all.... and the Lord reminded him.... that 'washing' applied to that which was dirty....i.e. the dusty feet in the sandals of that day which, though their bodies were clean, still their feet had become dirty while walking in the streets to the passover feast. So it is with the Christian life, we must faithfully be refreshed in the Word and apply it to the perfecting of our walk in Christ Jesus so that we turn from that which is unholy and un-Christ like, or repent from those areas of sin in our lives.
So though the saved person is counted as 'righteous' because the Lamb of God paid the penalty, and the Son of God became our righteousness.... still we do sin, though we desire not too..... and this daily cleansing of repentance, renews and restores the fellowship upon which we are dependant in our faith. He completed his work on the cross. But our walk of faith, or result of our faith which is action, continues until we either die or are transformed..... Til that moment, we are in the process of being transformed, putting off the old man and putting on the new, dying to ourselves daily, the process of santification.
So then, in answering the riddle... I would think the sinner sins because he is already a sinner. A sinner gets saved because he understands that he has sinned and that sin has consequences which he cannot pay nor live.... without accepting that God who holds him accountable is also his salvation in whom he must depend. So then a sinner becomes 'saved' having passed from the verdict of his sin unto death, through the substitutionary priceless death of Jesus, which miraculously presents him to the Father of Glory as spotless: He then, whos' own self-righteousness was as filthy rags, is now counted as 'righteous' in Jesus Christ: Clothed in the righteousness of the Lord Jesus Christ, he is no longer in bondage to be the servant or suffer the penalty of sin.... but he still might sin.... and when he does, he is admonished to confess and repent and not become bound thereby, so that he remains in right fellowship with God without self-condemnation separating him from the love of God.