One of the most unsettling things to me in my studies as a pastor was the realization that the doctrine of accountability doesn't exist in the bible. I'll explain:
1/

A doctrine is a church tradition that is not directly given in the sacred text, but is derived from our understanding of god and his intent. So a doctrine is often "Because we believe X, we also believe Y". It's one or two steps removed from textual authority.
2/

In the case of the doctrine of the age of accountability, there is no scripture that directly deals with the question of "what happens to children or people of limited mental capacities when they die?"
3/

In fact, the OT teachings of the brutal conquering of Canaan indicate that YHWH holds children (and even livestock) to the same level of culpability as their parents/owners.

This simply doesn't sit well with people. We don't want to believe in that deity.
4/

At some point, it became popular at the application level (not necessarily academia) to use a couple of passages to create a functioning doctrine that allows for the deity to show mercy to children and people with diminished mental capacities.
5/

In 2 Samual 12 King David speaks of his dead son in a hopeful manner that he will see him again someday. In Matthew 5 Jesus said "blessed are the pure in heart, for they will see God."

Those are the passages used to come up with the doctrine of the age of accountability.
6/

Luther, in a famous moment of passion (as he was often given to exhibit) carried the corpse of a teenage boy who had committed suicide to the church cemetery, dug his grave by hand, and tearfully roared to the shocked villagers watching "God MUST be merciful!" as he buried him
7/

This is how the doctrine of the age of accountability came to exist. People did not want to believe that god would condemn children or those with limited mental capacities to eternal damnation, and so they simply...created a doctrine to support it.
8/

This is no different than progressivism, which omits or explains away massive portions of the sacred text simply because they don't want to believe in cruel deity of the bible.
9/

This was another confirmation to me as a pastor and theologian that if I was going to follow the god of the bible, I had to come to terms with the fact that he was a sadistic asshole. Any other interpretation is lacking textual integrity.
10/

There is no direct indication anywhere in the scriptures that the god of the bible does not judge children or people with limited mental capacities for their sin the same as everyone else. The doctrine of original sin teaches that you're born a sinner, destined for hell.
11/end