I agree with you on the age of accountability concept but what about a person who’s an adult and hasn’t accepted Jesus in faith gets into an accident and suffers an acquired brain injury and is no longer intellectually competent? What do you think God’s word says about this? I know of no clear position in scripture.
There’s nothing in the Bible about this because it’s not a legitimate exception. The adult who suffers an accident and loses his or her intellectual capacity is no different from the one who dies suddenly. A sudden accident or death may take us by surprise but not the Lord. He knew all the days of our life before one of them came to be (Psalm 139:16) and arranges for each person to hear the gospel before disability or death makes it impossible for them to respond. Otherwise we could rightly accuse God of being unjust.
We’re not given any information about the span of our life. It could end at any moment. This is why the writer to the Hebrews said, “Today if you hear His voice do not harden your heart” (Hebr. 4:7). It may be your last chance.