Theophylact of Ohrid

These very names the apostle previously applied from the perspective of age to moral teaching, and now repeats them from the perspective of disposition, without interrupting his instruction.

Commentary on 1 John ↗

Clement of Alexandria

By strong "young men" John means those who have overcome their lusts.

From the Latin Translation of Cassiodorus ↗

Augustine of Hippo

"I write unto you, children." Whence children? "Because ye have known the Father. I write unto you fathers:" he enforceth this, and repeateth, "Because ye have known Him that is from the beginning." Remember that ye are fathers: if ye forget "Him that is from the beginning," ye have lost your fatherhood. "I write unto you, young men." Again and again consider that ye are young men: fight, that ye may overcome: overcome, that ye may be crowned: be lowly, that ye fall not in the fight. …

Ten Homilies on 1 John 2 ↗

Bede

I write to you, young men, because you are strong, etc. Consider again and again that you are young men, fight so that you may conquer, conquer so that you may be crowned. Be humble, lest you fall in battle.

Commentary on the Catholic Epistles ↗

Bonaventure

The gift of the Holy Spirit is given for overthrowing the powers of the air. Whence in John: "I write to you, young men, because you are strong, and the word of God abides in you, and you have overcome the evil one." This fortitude ought to be in every Christian and especially in the leader of the army of the Christian people.

Collationes de Septem Donis, Collation 5 ↗