When are we accountable?
Determining at what age an individual becomes spiritually accountable for what they think and do is IMPOSSIBLE for humans to determine. This is because only the Father knows the intentions of a person's heart and what they believe and understand. For a person to have a belief, there must be some understanding of what is to be believed.
"After John was put in prison, Jesus went into Galilee, proclaiming the good news of God. 'The time has come,' he said. 'The kingdom of God is near. Repent and believe the good news!'" (Mark 1:14-15, NIV).
To repent a person must have an understanding of what they are repenting of and what is the gospel. During one of his missionary journeys the apostle Paul and Silas were thrown into jail. While they were singing hymns at night a miraculous earthquake occurred that not only freed them from their shackles and cell but also broke the chains and opened the cells of ALL the prisoners! The guard of the prison was in such awe of the miracle that he ran to Paul and Silas, fell down before them trembling, and said:
“'Sirs, what must I do to be saved?' So they (Paul and Silas) said, 'BELIEVE on the Lord Jesus Christ, and you will be saved, you and your household.' Then they spoke the word of the Lord to him and to all who were in his house. And he took them the same hour of the night and washed their stripes. And immediately he and all his family were baptized. Now when he had brought them into his house, he set food before them; and he rejoiced, having believed in God with all his household." (Acts 16:29-34)
Note that the guard and his household had to understand and believe in the Gospel FIRST before they could be baptized. Peter said:
"'REPENT, and let every one of you be baptized in the name of Jesus Christ for the remission of sins; and you shall receive the gift of the Holy Spirit.'" (Acts 2:38).
What is needed for TRUE repentance?
Until a person becomes old enough, they do not have enough understanding to fully repent of their sins, begin to obey God, and with full knowledge of the consequences of their actions, be baptized. They also do not have the emotional maturity to deal with the sometimes narrow and difficult road a Christian must travel in this life. You can read the Bible from cover to cover and you will NOT find any example of babies or young children receiving baptism (as the Catholic Church promotes). Those who die in this life without receiving a full chance at salvation, no matter what age they are, will someday be resurrected and given the opportunity to receive eternal life.