God's Kingdom
Matthew 18 and Mark 10 states we must become like children to enter the Kingdom of Heaven. From these verses, spoken by Jesus, some conclude that God will immediately save infants and young kids who die.
"I assure you that unless you change and become LIKE children, you will never enter the Kingdom of heaven . . ." (Matthew 18:3, see Mark 10:14).
Notice that, in the above quote, Jesus did NOT say children are automatically saved upon death. He refers to two things that are needed in order to enter into eternal salvation. They are conversion and possessing the humility of someone who is very young. It is very clear that he was merely using very young humans as an example of the attitudes and attributes a person should have if they want God to save them.
Age of Accountability
Determining at what age children become spiritually accountable for what they think and do is impossible for humans to determine. This is because only God knows the intentions of a person's heart and he knows what people comprehend. For a person to have a belief, there must be some understanding of what is to be believed (see Mark 1:14 - 15).
Repentance Required
Children, in order to repent, must understand 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 evangelists and asked what it would take to become a Christian.
"Sirs, what must I (the guard) do to be saved?" They (Paul and Silas) answered, "Believe in the Lord Jesus . . . you and your family."
Then they preached the word of the Lord to him and to all the others in the house. At that very hour of the night the jailer took them and washed their wounds; and he and all his family were baptized at once (Acts 16:30 - 33).
Note that the guard and his household had to understand and believe in the Gospel first, then repent (both of which children cannot do), before they could be baptized.
The apostle Peter stated the above steps of repentance THEN baptism in his well-known message on Pentecost (Acts 2:38). These people made a conscious decision to repent of their sins and change how they live from disobeying God to being obedient to his words.
The acts listed above are virtually impossible for babies and young children to carry out. This means, in spite of the doctrinal statements and teachings of some churches, that baptizing infants and youngsters in order to "save" them is not a Scriptural principle!
Conclusion
Until children become old enough, they do not have enough understanding to save themselves by fully repenting of their sins, beginning 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 so that God can save them. 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.