As Christians, we are all at different stages in our Christian life. In his first letter, John addresses Christians in all the stages.
Even the youngest child of God is forgiven on account of Jesus Christ. There is no further qualification than faith in Christ for the forgiveness of sins. These children are also encouraged to know God as their Father, for this is their right and their joy.
After each referral to the children, the youngest Christians, John then moves to the oldest Christians, those who know the Father and his word. They are those who know the Creator, to one who is from the beginning and are acknowledged for their knowledge.
Finally John refers to those who are young, but strong in the faith. They are reminded not to rely on their own strength, but that their strength is found in Christ through his word, that they might overcome the evil one.
Of course, this only applies to those who have put their faith in Christ and have become children of God.