Objection 1. It would seem that Christ alone should have been baptized with the baptism of John. For, as stated above (Article 1), "the reason why John baptized was that Christ might receive baptism," as Augustine says (Super Joan., Tract. xiii). But what is proper to Christ should not be applicable to others. Therefore no others should have received that baptism.
Objection 2. Further, whoever is baptized either receives something from the baptism or confers something on the baptism. But no one could receive anything from the baptism of John, because thereby grace was not conferred, as stated above (Article 3). On the other hand, no one could confer anything on baptism save Christ, who "sanctified the waters by the touch of His most pure flesh" [Mag. Sent. iv, 3]. Therefore it seems that Christ alone should have been baptized with the baptism of John.
Objection 3. Further, if others were baptized with that baptism, this was only in order that they might be prepared for the baptism of Christ: and thus it would seem fitting that the baptism of John should be conferred on all, old and young, Gentile and Jew, just as the baptism of Christ. But we do not read that either children or Gentiles were baptized by the latter; for it is written (Mark 1:5) that "there went out to him... all they of Jerusalem, and were baptized by him." Therefore it seems that Christ alone should have been baptized by John.
On the contrary, It is written (Luke 3:21): "It came to pass, when all the people were baptized, that Jesus also being baptized and praying, heaven was opened."
I answer that, For two reasons it behooved others besides Christ to be baptized with the baptism of John. First, as Augustine says (Super Joan., Tract. iv, v), "if Christ alone had been baptized with the baptism of John, some would have said that John's baptism, with which Christ was baptized, was more excellent than that of Christ, with which others are baptized."
Secondly, because, as above stated, it behooved others to be prepared by John's baptism for the baptism of Christ.
Reply to Objection 1. The baptism of John was instituted not only that Christ might be baptized, but also for other reasons, as stated above (Article 1). And yet, even if it were instituted merely in order that Christ might be baptized therewith, it was still necessary for others to receive this baptism, in order to avoid the objection mentioned above.
Reply to Objection 2. Others who approached to be baptized by John could not, indeed, confer anything on his baptism: yet neither did they receive anything therefrom, save only the sign of penance.
Reply to Objection 3. This was the baptism of "penance," for which children were not suited; wherefore they were not baptized therewith. But to bring the nations into the way of salvation was reserved to Christ alone, who is the "expectation of the nations," as we read Genesis 49:10. Indeed, Christ forbade the apostles to preach the Gospel to the Gentiles before His Passion and Resurrection. Much less fitting, therefore, was it for the Gentiles to be baptized by John.
Source: Summa Theologica (New Advent)