4 “He must increase, but I must decrease.” What is this? He must be exalted, but I must be humbled. How is Jesus to increase? How is God to increase? The perfect does not increase. God neither increases nor decreases. For if He increases, He is not perfect; if He decreases, he is not God. And how can Jesus increase, being God? If to man's estate, since He deigned to be man and was a child; and, though the Word of God, lay an infant in a manger; and, though His mother's Creator, yet sucked the milk of infancy of her: then Jesus having grown in age of the flesh, that perhaps is the reason why it is said, “He must increase, but I must decrease.”
But why in this? As regards the flesh, John and Jesus were of the same age, there being six months between them: they had grown up together; and if our Lord Jesus Christ had willed to be here longer before His death, and that John should be here with Him, then, as they had grown up together, so would they have grown old together: in what way, then, “He must increase but I must decrease”? Above all, our Lord Jesus Christ being now thirty years old, does a man who is already thirty years old still grow?
From that same age, men begin to go downward, and to decline to graver age, thence to old age. Again, even had they both been lads, he would not have said, “He must increase,” but, We must increase together. But now each is thirty years of age. The interval of six months makes no difference in age; the difference is discovered by reading rather than by the look of the persons.
Source: Tractates on the Gospel of John (New Advent)