Salvation is not based on intellect or understanding. I don't believe in infant baptism. Infant baptism suggests salvation is at the hands of people. It is not. It's at the hands of God. I know I am saved, because I hear His voice. God's children, of any age, can hear His voice.
Jesus said this: "My sheep hear My voice, and I know them, and they follow Me" (John 10:27).
Luke 18:15-17 People were bringing even infants to him that he might touch them; and when the disciples saw it, they sternly ordered them not to do it. But Jesus called for them and said, "Let the little children come to me, and do not stop them; for it is to such as these that the kingdom of God belongs. Truly I tell you, whoever does not receive the kingdom of God as a little child will never enter it."