God could have created Jesus as an adult man, like he did for Adam. He came, instead, as a babe in Bethlehem. Why? In describing the work of Jesus, Johann Gerhard cites favorably the Early Church Father Irenaeus (130-202) who gives a good reason.
Christ sanctified every age through that which was after His likeness, for He came to save all people through Himself; all, I say, who through Him are reborn to God as infants, as well as children and youth and the elderly. Therefore he came through every age, having become an infant among infants, sanctifying infants; as a child among children, sanctifying those having this very age; and at the same time, being an example of piety and righteousness and subjection for them, Being a youth to the youths, as an example to the young and sanctifying them for the Lord, He was also an elder among the elderly in order to be a perfect master among all people, not only according to the explanation of the truth but also according to His age, sanctifying also the elderly. (Irenaeus, bk. 2, ch. 39, p. 137)
Gerhard, Johann Theological Commonplaces: Exegesis IV on Christ 310