Baptism did not start with John the Baptist. It was the practice of the Jews to baptise converts as much as five centuries before John the Baptist was born. John was simply building on what was already established.
The terms for Jewish baptism are Tevilah and Netilat Yadayim. Tevila is full body immersion in water, whereas Netilat Yadayim is the pouring of water over the hands with a cup.
The Bible does not describe the ritual of either Tevilah or Netilat Yadayim; although it is prefigured by the sprinkling of water in Ezekiel 36:25, and the sprinkling of blood in Leviticus 14:7 and Leviticus 16:14; and is prescribed in Leviticus 14:8-9, Leviticus 15:5, Leviticus 17:15, Leviticus 22:6, and Numbers 19:7. Matthew 23:25-26 and Luke 11:39 also refer to Tevilah; as does Mark 7:1-5, which also refers to Netilat Yadayim. Actually, the Greek words used for Netilat Yadayim and Tevilah in Mark 7:4, βαπτισωνται (baptisontai) and βαπτισμους (baptismous), have the same root as the Greek word βαπτισμα (baptisma), from which we get the English word baptism.
In addition to baptising utensils used for food, Jewish men and women would themselves be baptised to achieve ritual purity after becoming ritually unclean by a number of conditions, such as menstruation and childbirth, and coming into contact with a dead body or other ritually unclean object. Some time between 200 to 500 years before the birth of Jesus and John, baptism became a part of the traditional procedure for conversion to Judaism. A family coming into the Jewish Faith would all be baptised, and all the males would be circumcised. This would include the entire family, including babies.
From the beginning of Christianity, the Sacrament of Baptism, like Jewish baptism, was also performed on infants. It was not until the Petrobrusians heresy in the eleventh and twelfth centuries that the validity infant baptism was questioned. The way the founder of this heresy, Peter of Bruys, interpreted the four Gospels (he rejected the rest of the New Testament, as well as the Old Testament), baptism had to be preceded by personal faith.
If Christian baptism differed from Jewish baptism by excluding infants and young children, it would have been documented somewhere. Instead, the Book of Acts records that whole family were baptised. In Acts 16:15, Paul uses the word οικος (oikos), which means one's household, or one's entire property; and in Acts 16:34, he uses the word πανοικει (panoikei), which quite specifically means with all his house, or with his whole family.
In fact, the Bible does not contain any directions on baptism other than that it is done with water and in the Name of the Father and of the Son and of the Holy Spirit. The earliest recorded instructions on baptism are found in the Didache or Teaching of the Twelve Apostles, which dates back to between 65 and 90 A.D. and may have as least some of the Apostles as authors. The Didache states the following:
In regard to Baptism - baptize thus: After the foregoing instructions, baptize in the name of the Father, and of the Son, and of the Holy Spirit, in living water. If you have no living water, then baptize in other water; and if you are not able in cold, then in warm. If you have neither, pour water three times on the head, in the name of the Father, and of the Son, and of the Holy Spirit. Before the Baptism, let the one baptizing and the one to be baptized fast, as also any others who are able. Command the one who is to be baptized to fast beforehand for one or two days. (Didache 7:1-4)
This is where the physical act of John's baptism may have differed from Christian baptism. John's baptism likely imitated the baptism of Jewish converts, which was Tevilah, and was by total immersion. The Didache testifies that the Apostles also baptised in the mode of Netilat Yadayim, which was the pouring of water.
Nevertheless, Christian baptism was prefigured by John's baptism, which was prefigured by Jewish baptism.