1 Peter 3:21 tells us that baptism is the answer of a good conscience toward God. How can a child who does not profess faith in God answer him with a good conscience?
Heb. 11:20, "By faith Isaac blessed Jacob and Esau concerning things to come."
That doesn't say that Issac baptized Esau or Jacob. It seems to me 1 Peter is setting out a definition for what baptism is and is for, and an unbelieving 18year old (which the OP was concerned with) cannot fulfil that definition.