The original sin was the transgression of Adam and Eve to partake of the fruit of the tree of knowledge of good and evil. The two of them were immortal before partaking of the fruit but afterwards they became mortal. Thus they were subject unto death. They also gained a knowledge of good and evil from that time forth. Prior to partaking of the fruit, Adam and Eve were in a state of innocence not knowing good from evil.
The Church of Jesus Christ of Latter-day Saints holds to the following
Article of Faith:
2 We believe that men will be punished for their own sins, and not for Adam’s transgression.
We believe that Jesus Christ atoned for original sin. Thus none of us who are born on this earth will be punished for Adam's transgression. This means that even though we are born as mortals, all mankind, whether good or evil will participate in the resurrection of the dead. Thus there will be a resurrection of both the just and the unjust.
Acts 24:15
15 And have hope toward God, which they themselves also allow, that there shall be a resurrection of the dead, both of the just and unjust.
John 5:29
29 And shall come forth; they that have done good, unto the resurrection of life; and they that have done evil, unto the resurrection of damnation.
This also means that we are not held accountable for Adam and Eve's transgression or any other person's sins but only for our own sins.
Moses 6:54
54 Hence came the saying abroad among the people, that the Son of God hath atoned for original guilt, wherein the sins of the parents cannot be answered upon the heads of the children, for they are whole from the foundation of the world.
Because of the atonement of Jesus Christ, little children are alive in Christ and cannot sin until they reach the age of accountability.