For John, Jesus' ministry begins with his role in a wedding celebration. It is not in any obvious way a fulfillment of scripture. We are never fully prepared. That's the nature of life, of commitments, and of sacraments. Newborn babies don't get a vote about staying in the womb.
Infants getting baptized have no idea what kind of life they're being signed up for.
I wonder if adults getting baptized know. Certainly a couple getting married have no idea what they're getting into-not even if they've lived together for years. It's not a contract laying out expectations of fees paid for services rendered and how to resolve disagreements when they arise and how to dissolve the contract when the partners can no longer work together. At least in its sacramental Christian form it is not. It is a covenant, a pledge, a sacred vow made before God and the witness of the community.