There are several ways to explain the text. The most simple is to consider that John is writing to three groups of Christians, little children, young men, and fathers. It is not dogmatic, but Barnes suggests years to approximate 20 & below, 20-40, & above 40.
The little children (note, there are two different words translated in this way in 12a and 13b which may or may not have any significance), are told by John he writing was “because your sins are forgiven you” and “because you have known the Father.” John was certainly writing to young Christians to mature them in faith.
Young men, “because you have overcome the wicked one,” and “because you are strong and the word of God abides in you, and you have overcome the wicked one.” This indicates valiant and strong Christians, who receive strength from John’s words.
Fathers, “because you have known Him who is from the beginning.” This is repeated twice, a confirmation of their long held faith, close acquaintance, and trust in God the Father.
