Here is what you said.
There is no "age of reason" mentioned or implied in the Bible.
I'm saying God never teaches that people are automatically saved until some unspecified "age of reason." You are free to provide scripture and prove me wrong.
I never provided you with any Scripture to prove you wrong, so you are right, and God does hold sin against babies. They have to go to hell because they were never saved. Your logic, not mine.
So, you are stating that since you can find no scripture for the "age of reason" theory, therefore God sends infants to hell. That is your conclusion.
My conclusion is that all humans are saved or cursed by God's choice to either extend grace or not extend grace. The age doesn't matter. The only thing that matters is what God decides.
The question is: What does God decide about infants who are not capable of expressing a discernable, God given, faith? Does God extend grace?
Here is what we know from scripture when David's infant child died.
2 Samuel 12:19,23 But when David saw that his servants were whispering together, David understood that the child was dead. And David said to his servants, “Is the child dead?” They said, “He is dead.”
But now he is dead. Why should I fast? Can I bring him back again? I shall go to him, but he will not return to me.”
When God records David's response, is David wrong in his belief about seeing his son some day after David dies?
I believe God recorded this to give us a glimpse of God's decision to be gracious to infants whom he has allowed to die. Can I speak definitively? No. I must live by faith in what I read and have concluded.
But, the "age of reason" theory, removes God as the one who determines who is redeemed. Instead the "age of reason" theory places the responsibility for salvation upon the rational decision making of the human being with God being a passive observer who does not make a choice at all. The burden for salvation lies entirely on the human's discernment skills.
What I reject is the syncretist teaching of human choice superceding God's authority to choose. I reject your soteriological assumption.