I write unto you, little children, because your sins are forgiven you for his name's sake. I write unto you, fathers, because ye have known him that is from the beginning. I write unto you, young men, because ye have overcome the wicked one. I write unto you, little children, because ye have known the Father. I have written unto you, fathers, because ye have known him that is from the beginning. I have written unto you, young men, because ye are strong, and the word of God abideth in you, and ye have overcome the wicked one. (1 John 2:12-14 KJV)
John writes to children, young men and fathers. This shows that the gospel is for people of all ages. It also speaks to the varying degrees of spiritual maturity among those who are Christians.
Those who are "children" in the faith often worry about whether or not their sins are actually forgiven in Christ or on the other hand they may revel, almost in a giddy way, in the knowledge of having their sins forgiven. They are immature in their faith, needing lots of guidance. So John assures them that their sins are forgiven and adds "for his name's sake", speaking of Christ, so as to eliminate any other way to forgiveness.
Those who are "young men" in the faith have matured beyond the spiritual child stage and are strong in the faith, having overcome the temptations of the wicked one and they have been spending time in God's word regularly. They are not as prone to the fits and starts of the spiritual childhood stage.
Those he addresses as "fathers" refer to those who are spiritually mature in their faith. They have settled into a quiet confidence in Him who has been from the beginning, having experienced His faithfulness in all seasons of their spiritual growth. This is not to say they are perfect or have reached a sinless perfection. None on this earth will reach that this side of heaven.
It is good to realize that there are varying degrees of spiritual maturity among believers, not so I can categorize and put everyone in a spiritual box or compare myself to others to pull myself up in self righteousness, but rather so I might treat others with mercy, love and understanding.