“Accountability” is not a term explicitly found in Scripture. Rather it is an implicit and abstract concept that is, of course, biblical. This is obvious because of the many references to the various judgments and because of passages like Acts 17:31 and Romans 14:12. The last passage uses the English term, “give an account.” But I know of no passage that specifically speaks of an age of accountability even in an abstract way. However, as with other doctrines like the trinity that are not specifically spelled out, many theologians and Bible students have come to the conclusion that there is an age of accountability, though it undoubtedly varies with individuals.
Some believe the age of accountability could have been around 20 years old because this was the age when young men in Israel became accountable to serve in the army of Israel.
Others believe the age of accountability to be around 12 or 13 due to this being the age when Jesus went up to Jerusalem with his parents and was found in the temple discussing the Law and asking questions. This was also the normal age for being received into Judaism as a “son of the law,” which would make him a full member of the religious community.
But all of this is conjecture since it is nowhere actually stated in Scripture. I think, from what I can gather (and I am certainly no expert on this), the reasoning behind believing in an age of accountability boils down to a couple of issues: First, people are accountable for their response to the witness of God in creation (Rom. 1:18f) and to the witness of God in the Bible and in the person of Christ (Acts 17:31). But second, since that response is based on one’s ability to comprehend and respond to the message, a person becomes personally accountable when he or she reaches a point where they have the spiritual and mental facility to grasp the issues. This does not mean they are not sinful, but only that they have not reached a place where they can understand.
Of course all men are born spiritually dead (without the natural ability to respond) and under the condemnation of sin, but Christ seeks to draw all men to Himself through the ministry of the Spirit and He bore that condemnation for man by His death on the cross. The accountable issue then is turning from self trust in good works or from apathy and a denial of accountability to God to trust in Christ (John 16:8f). It is significant that the Spirit’s ministry of convicting and giving demonstrable proof to men relates to their trust or rejection of Christ.
John 16:8-9. And He, when He comes, will convict the world concerning sin, and righteousness, and judgment; 9 concerning sin, because they do not believe in Me; (emphasis mine).
“Whoever believes in the Son has eternal life, but whoever rejects the Son will not see life, for God’s wrath remains on him.” (NIV)
But when does this accountability begin? Again, since the primary issue seems to be response to the revelation God has given, personal accountability would vary and depend on one’s ability to understand the most fundamental issues. In some, this might be very young, while in others it might be much older. I had a Christian friend who was a doctor and worked with severely retarded people. He said some seemed to grasp some of the issues but others simply could not. However, since Christ died for the world and paid the penalty for sin (1 John 2:2) and since the issue is one of responding to the revelation of God in creation (God consciousness) and in Christ, the issue seems to boil down to that point in time when one can understand and respond to the convicting work of the Spirit as described in John 16.