Mormons baptize at the age of eight years and older, because this is the age when children can understand between right and wrong, decide to choose one or the other, and understand the concept of repentance. Mormons call this “the age of accountability.” Until children are eight years old, their parents are responsible for their behavior. Little children are born innocent. The atonement of Jesus Christ covered the sin of Adam, and so there is no “original sin.” Little children (and adults who never attain accountability due to mental retardation) who die before age eight are saved and exalted to the highest level of heaven, called the Celestial Kingdom. Mormons call this being “saved in innocence.”
In the Book of Mormon, it says,
“…the word of the Lord came to me by the power of the Holy Ghost, saying: Listen to the words of Christ, your Redeemer, your Lord and your God. Behold, I came into the world not to call the righteous but sinners to repentance; the whole need no physician, but they that are sick; wherefore, little children are whole, for they are not capable of committing sin; wherefore the curse of Adam is taken from them in me, that it hath no power over them; and the law of circumcision is done away in me.
“And after this manner did the Holy Ghost manifest the word of God unto me; wherefore, my beloved son, I know that it is solemn mockery before God, that ye should baptize little children. Behold I say unto you that this thing shall ye teach—repentance and baptism unto those who are accountable and capable of committing sin; yea, teach parents that they must repent and be baptized, and humble themselves as their little children, and they shall all be saved with their little children.
“And their little children need no repentance, neither baptism. Behold, baptism is unto repentance to the fulfilling the commandments unto the remission of sins. But little children are alive in Christ, even from the foundation of the world; if not so, God is a partial God, and also a changeable God, and a respecter to persons; for how many little children have died without baptism!
“For awful is the wickedness to suppose that God saveth one child because of baptism, and the other must perish because he hath no baptism. Wo be unto them that shall pervert the ways of the Lord after this manner, for they shall perish except they repent. Behold, I speak with boldness, having authority from God; and I fear not what man can do; for perfect love casteth out all fear.
“And I am filled with charity, which is everlasting love; wherefore, all children are alike unto me; wherefore, I love little children with a perfect love; and they are all alike and partakers of salvation. For I know that God is not a partial God, neither a changeable being; but he is unchangeable from all eternity to all eternity. Little children cannot repent; wherefore, it is awful wickedness to deny the pure mercies of God unto them, for they are all alive in him because of his mercy. And he that saith that little children need baptism denieth the mercies of Christ, and setteth at naught the atonement of him and the power of his redemption (Moroni 8:7-20).
The Prophet Joseph Smith went one step further in talking about little children who die before they are accountable. He said that they will be resurrected to the age that they were when they died, and that their bereft mothers will then be able to raise them in heaven to adulthood. This sounds like a strange doctrine, but heaven is for the reuniting of loved ones and the healing of all wounds. For any mother who has lost a child, this doctrine provides more hope and healing than any other.