We sin because we have a sin nature from conception.
But we are not held accountable for that sin until it comes alive in us, like a disease going active after dormancy. Paul could not be clearer about this in Romans 7.
Are we condemned to death under sin because of our nature or because of our sins? In a funny way, it doesn't matter, for Jesus was the one sacrifice for all. The unintentional/unknown sins are just as much covered under OT law as all other sins, and Jesus died for the unknown sins, too. So the babies, who are not yet old enough to sin volitionally, but who nevertheless are operating according to their sin natures, are saved regardless.
Here is an example from everyday life. Most the people here know I have a profoundly retarded son, Chris. He had encephalitis when he was 3 and today, although physically active, has a 19 IQ, cannot speak, and will always be in diapers. He is seventeen and taller than I am now. It makes life interesting.
We have to lock Chris in his bedroom at night. If not, he will wake up whenever he happens to wake up and go have a really good time in the kitchen. We woke up, one morning a couple of years ago, to about 3 pounds of Tang powder all over the kitchen, the wrapper of a loaf of bread which had been cheerfully shared with the dogs, a melted plastic bag in the microwave, and ashes from the wood stove (I thank God we had not used it the evening before and everything was cool) all over the hearth and family room.
Could I hold him accountable for a mess that took me, literally, several hours to clean up? No, I could not. Chris knew I was upset, but he didn't connect that with anything he had done. That sort of connection comes hard to him. But I still had to clean up.
Now, if my seventeen year old daughter, who gets over a 3.0 average in school and is very verbal and understands the rules of the house and the reasons behind them quite well, were to scatter a few pounds of Tang across the kitchen, melt a plastic bag in the microwave, share a loaf of bread with the dogs, and scatter ashes all over the place -- she would be grounded until she was about 95 or so!
In both cases, however, I would have to clean up. In both cases she did (would) help, while Chris could do nothing.
Now we can't help Jesus clean up after our messes, but whether we are in a position to be held accountable for what we do or not, He has cleaned up.
As we grow we can follow our rebellious nature or allow Christ to put it to death and grant us a new nature -- thus being born again.
That new nature WANTS to keep the kitchen clean, although spills and accidents will still occur at times.
The kitchen, then, can get dirty one of three ways:
1. Via Chris -- he likes Tang, and bread, and the lights in the microwave and the warmth of the wood stove. So he tried to help himself to all without having a clue what he was doing. Unknown sin.
2. Via direct, conscious rebellion -- if Bianca had done the same thing, knowing what she was doing and either doing it out of spite/anger, or just because she felt like it ('if it feels good, do it!'). Volitional sin, which demands accountability.
3. Accidental mess despite trying not to -- that's me. I don't want to make a mess but I manage to on a rather frequent basis anyway.
Jesus has cleaned it all up. That's the point. Whether or not we choose to accept that gift of cleanliness for ourselves is the choice we have.
As a believer in the straightforward reading of the Bible, I am, of course, a YEC. This same straightforward reading says that God truly does love the WORLD (or all men), John 3:16, and that He truly is not willing that ANY should perish (2 Peter 3) and that Jesus truly was the priest who "offered for all time one sacrifice for sins" (Heb. 10:12). So why are only a few chosen? Because most prefer themselves and refuse the gift of salvation.
Those of us who accepted did nothing to earn that gift, establish it, wrap it, keep it -- it is all a matter of grace. But the Bible makes it very clear that a man can suppress the truth that he knows (Romans 1), thereby starting to follow a lie and will end up in death and degradation.
Or a man can seek. Jesus told us to FIND the narrow gate. That is an activity. That has nothing to do with having constructed the gate, opening it, or building the path. All that is already done. But we are told to ask, seek, knock.
I can't get away from those verses.
Does it mean a man can actually do anything about his own salvation? No, it does not. That is something Jesus established for us. But nowhere in the Bible do I read that God so hated most of the world that He denied them forgiveness and salvation from before their conception. The Bible teaches exactly the opposite, in combination with the freedom of man to choose one way or the other.
Is God surprised by who chooses yea or nay? Not at all. He is omniscient, knowing the end from the beginning. He is big enough to not only know, but to still allow us the freedom to choose.
In conjunction with this, the babies, and Chris, are safe in God's hands forever (until they grow up, which most babies do and Chris never will), because ALL their sins are unknown and still covered by Christ's sacrifice.
So yes, we sin because we are subject to our sin natures. But, again Romans 7:7-11, a baby or child is alive in Christ until that sin comes to life and kills, or separates, him or her. There is no other meaning that can be taken from Paul's words there. Apart from the law, sin is dead. If sin is dead, sin cannot hurt a person. It is only when, using the example of coveting, Paul KNEW what the law was that he, in his original perverse nature, found himself coveting more and more as a matter of desire, thus dying spiritually, or being separated from God.
And, finally, yes, all have sinned and come short of the Glory of God. That is never in question. But little ones are not, and cannot be, held accountable. And Jesus has taken care of them, too, with His sacrifice.