1 John 2:13

I write unto you, fathers
Not merely in age, though they might be men in years who are here intended, or only with respect to their long standing in the church, which might be the case; though persons may be in years, and of a long standing in the church, and yet be children in knowledge and experience: but here it designs such, who, in comparison of others, were perfect, and were spiritual, and judged all things; had a well informed and established judgment in divine things, and were, in understanding, men, fathers, and not babes in Christ; so the Jews used to call their men of wisdom, and knowledge, and understanding, (twba) , "Abot", "fathers". Hence there is a whole treatise in the Misna called Pirke Abot, which contains the apophthegms, wise sayings, and sentences of their fathers, or wise men. Now the apostle writes the new commandment of love, and urges it on these, for this reason,

because ye have known him [that is] from the beginning;
either God the Father, who is from everlasting to everlasting, the Ancient of days, the eternal I AM, whom to know is life eternal; whose everlasting love to them, whose covenant of grace with his Son for them, before the world was, and the ancient transactions, and settlements of his grace on their account, they were acquainted with: or Jesus Christ, the Logos or Word, which was from the beginning, who existed from all eternity, as a divine person, as the Son of God, co-eternal with the Father; as the eternal choice made in him, and the everlasting covenant with him show; and who in his office capacity, as Mediator, was set up from everlasting; and who, with respect to the virtue of his blood, righteousness, and sacrifice, was from the beginning of the world, and was the same yesterday, today, and for ever; it being by his blood that all the patriarchs, from the beginning of time, were pardoned, and by his righteousness they were justified, and by his grace they were saved; all which, respecting the antiquity of Christ's person, office, and grace, was known to these fathers: they knew him, so as to approve of him, trust in him, and appropriate him to themselves, and which obliged them to the new commandment of love, not only to God and Christ, but to one another; and the reason here given, engaging to it, is exceeding suitable to their character, it being what fathers and aged men delight in, even ancient things, to call them to remembrance, to talk of them as things well known unto them; but nothing is more ancient than what is here instanced in, and nothing so honourable and profitable to know as this, or to be gloried in; and therefore the argument from hence to love those that belong to him, who is the everlasting Father, is very strong and forcible.

I write unto you, young men;
who are warm and zealous for God, for his cause and interest, for the glory of a Redeemer, for his truths and ordinances; and are lively in the exercise of grace, and fervent in the discharge of duty; and are active, diligent, and industrious, always abounding in the work of the Lord; and are strong and robust, able to go alone, to walk by faith, being strong in it, and in the grace that is in Christ, and do not need the staff that old age does, nor the hand to lead and teach to go, as children do: to these the apostle writes the new commandment of love, for this reason,

because ye have overcome the wicked one;
Satan, who is eminently so, being the first that was, and the worst that is so; for he is wickedness itself, he is wholly, entirely, immutably, and unalterably wicked; and his whole work and employment is in wickedness. Now these young men had overcome him, not only in Christ their head, who has spoiled him, destroyed him, and led him captive in triumph, in whom they were more than conquerors; but in themselves, through the power of divine grace, holding up, and making use of the shield of faith against him, whereby they quenched his fiery darts, and got the victory over him: and this is also said in perfect agreement with the character of young men, who are apt to glory in their strength, and are fond of getting the advantage, or a victory over others; and which is used to teach such as are so in a spiritual sense, not to glory in their strength, but in the Lord; and to love him whom they know, and whose lovingkindness is exercised towards them, and in Christ; and to love him through whom they get the victory, and to bear the infirmities of weaker saints, to whom they should be strongly affected.

I write unto you, little children;
or babes in Christ, such as were newborn babes, just born again, not able to go alone, or walk by faith, but were dandled on the knee, and lay at the breasts of divine consolation: could speak but stammeringly, and not plain, it being as much as they could do to say "Abba", Father. To these the apostle writes, and urges the new commandment of love, for this reason,

because ye have known the Father:
the Father of Christ, and him, as their Father in Christ, under the witnessings of the spirit of adoption; so as, in some good measure, to hope and believe he was their Father, and to love, honour, and obey him as such, to apply to him for whatever they stood in need of, and always to put themselves under his care and protection: and a consideration of this their relation to him, and interest in him, is a strong and prevailing argument why they should not only love him, their Father, and Christ, who is begotten of him, but also all the saints, who are the children of this their Father, and their brethren; and very aptly does the apostle mention their knowledge of the Father as suitable to their age and character, it being one of the first and most necessary things for a child to know.