When an individual has fully and willingly submitted their heart to God, desiring His will to be done in their life, they choose to make an outward expression of this relationship with God. Water baptism, full immersion into water, is merely a symbol of what has already taken place in their heart. Jesus’ baptism was an example for us. “And it came to pass in those days, that Jesus came from Nazareth of Galilee, and was baptized of John in Jordan. And straightway coming up out of the water, he saw the heavens opened, and the Spirit like a dove descending upon him.” ( Mark 1:9,10) In this example, Christians desire to be obedient to Jesus’ instruction to follow in his footsteps by “taking up their cross and following him.” (Mark 10:21) The scriptures present baptism as a beautiful picture by lowering a person into the water, that is the death of the old, and arising from the water to newness of life. Baptism under this arrangement provides a beautiful privilege of demonstrating a spiritual desire to walk anew with our Lord. The word “baptism” in Greek as used in the scriptures is “baptizo” meaning to “make whelmed or fully wet.”
The act of baptism is secondary to the spiritual matters that are sincerely and maturely considered prior to baptism. The commitment made in complete submission to the will of God is one not to be taken lightly.
This scriptural arrangement provided for followers of Christ requires a mature person who is capable of learning, reasoning and making decisions.
In response to the second part of your question, with all this considered, the true intention of baptism would not be appropriate for a baby. An infant is neither capable of this level of reasoning and decision making, nor would it be appropriate to fully immerse a baby. Furthermore, one person cannot determine the beliefs and desires of another person. The decision to give up one’s will in full acceptance of God’s will to be done in their life is impossible for an infant and symbolizing submission by water baptism would be completely inappropriate.