There appears to be little Biblical evidence for an age accountability outside of David's comments in 2 Samuel 12:21-23. After the death of his son born out of adultery to Bathsheba he says, "I will go to him, but he will not return to me." David appears to understand that he will be with his infant son once again.
This is the best paragraph of the dictionary entry on "Age of Accountability." The article is weak, has several editorial errors, and tries to please everyone, it seems, in the last paragraph. Not terribly satisfying.
How would you approach this subject?