Objection 1. It would seem that Christ did not begin to work miracles when He changed water into wine at the marriage feast. For we read in the book De Infantia Salvatoris that Christ worked many miracles in His childhood. But the miracle of changing water into wine at the marriage feast took place in the thirtieth or thirty-first year of His age. Therefore it seems that it was not then that He began to work miracles.
Objection 2. Further, Christ worked miracles by Divine power. Now He was possessed of Divine power from the first moment of His conception; for from that instant He was both God and man. Therefore it seems that He worked miracles from the very first.
Objection 3. Further, Christ began to gather His disciples after His baptism and temptation, as related Matthew 4:18 and John 1:35. But the disciples gathered around Him, principally on account of His miracles: thus it is written (Luke 5:4) that He called Peter when "he was astonished at" the miracle which He had worked in "the draught of fishes." Therefore it seems that He worked other miracles before that of the marriage feast.
On the contrary, It is written (John 2:11): "This beginning of miracles did Jesus in Cana of Galilee."
I answer that, Christ worked miracles in order to confirm His doctrine, and in order to show forth His Divine power. Therefore, as to the first, it was unbecoming for Him to work miracles before He began to teach. And it was unfitting that He should begin to teach until He reached the perfect age, as we stated above, in speaking of His baptism (39,3). But as to the second, it was right that He should so manifest His Godhead by working miracles that men should believe in the reality of His manhood. And, consequently, as Chrysostom says (Hom. xxi in Joan.), "it was fitting that He should not begin to work wonders from His early years: for men would have deemed Incarnation to be imaginary and would have crucified Him before the proper time."
Reply to Objection 1. As Chrysostom says (Hom. xvii in Joan.), in regard to the saying of John the Baptist, "'That He may be made manifest in Israel, therefore am I come baptizing with water,' it is clear that the wonders which some pretend to have been worked by Christ in His childhood are untrue and fictitious. For had Christ worked miracles from His early years, John would by no means have been unacquainted with Him, nor would the rest of the people have stood in need of a teacher to point Him out to them."
Reply to Objection 2. What the Divine power achieved in Christ was in proportion to the needs of the salvation of mankind, the achievement of which was the purpose of His taking flesh. Consequently He so worked miracles by the Divine power as not to prejudice our belief in the reality of His flesh.
Reply to Objection 3. The disciples were to be commended precisely because they followed Christ "without having seen Him work any miracles," as Gregory says in a homily (Hom. v in Evang.). And, as Chrysostom says (Hom. xxiii in Joan.), "the need for working miracles arose then, especially when the disciples were already gathered around and attached to Him, and attentive to what was going on around them. Hence it is added: 'And His disciples believed in Him,'" not because they then believed in Him for the first time, but because then "they believed with greater discernment and perfection." Or they are called "disciples" because "they were to be disciples later on," as Augustine observes (De Consensu Evang. ii).
Source: Summa Theologica (New Advent)