"I am writing to you, little children, because your sins are forgiven you for His name's sake. I am writing to you fathers, because you know Him who has been from the beginning, I am writing to you, young men, because you have overcome the evil one. I have written to you, children, because you know the Father. I have written to you, fathers, because you know Him who has been from the beginning. I have written to you, young men, because you are strong, and the word of God abides in you, and you have overcome the evil one."
One of John's main concerns throughout the epistle is fellowship. Here, we see him doing what he can to build fellowship in his readers.
Here is how he achieves that:
First, he points out three groups that traditionally would be separated by lifestyle. Children, young men, fathers.
Next he provides a common point of interest in them. They all relate to the Father, God.
Finally, he encourages each one specifically.
Now all three groups understand that they are all fighting for the same kind of righteousness and growth.
If you seek righteousness - seek others to fight with you.