I think you are on the right track, Sophie. There IS an age of accountability in the Bible, or indicated by the Bible, as Mister Emu stated. It is twenty years old.
I noticed something raising our children which made me understand why it was twenty. When a child does something, he does not do it because it is right for the sake of right or wrong for the sake of wrong. He does what he does to please his parents, or his peers, or to impress someone, or to avoid punishment, etc. Teens, although they are becoming adults, are essentially no different. They do what they do to please others, to be accepted or popular, for allowances, grades, or other gratification. There does seem to be the occasional teen who has a keen sense of right and wrong, but for the most part, this does not affect decision-making until they hit the very late teens or early twenties. Then something is done simply because it is right. Right for the sake of right, irregardless of others.
We know that the teen brain is literally being 're-wired.' Neurological connections are being broken and re-established. Two of our sons checked out to some distant planet about the age of fourteen and started returning in the late teens. It was fascinating! Frustrating in the extreme at times, but definitely fascinating!
Of course we have to discipline our kids while they are growing up. We have to teach them what accountability means, and if we mess that one up, we are really messing their lives up but good. However God evidently does not hold any but the sentient adults (20 and up and mentally capable) accountable in a spiritual sense.
Does this mean we should abort babies to get them to heaven for sure? God forbid! That is murder. Sure, the babies may be safe with the Lord, but what about the doctors, nurses, mothers and fathers who are now guilty of murder? The babies belong to the Lord no matter, but let us love our neighbors as ourselves and care violently for those who do not realize that what they are doing is murder! They are the ones in jeopardy.
But about the babies -- God does not create a life for no purpose. Each one of our children has a reason to be born and to be alive. There are no accidental conceptions where God is concerned. He is the author of life, and He knows what each life is for.
Let us not try to take over His role in deciding who should live and who should die, with the exception of following the law He gave to all of us through Noah -- murder requires capital punishment.
But that has to be done legally, please, and not by shooting an abortion doctor or blowing up a clinic. You see, it is not the babies who are in danger of hell, but the parents and medical staff, and killing one of them or blowing up a clinic is going to do nothing to help save any of them in the long run.