After the believer is baptized, the pastor should offer him or her continuing discipleship training so that the new Christian may grow in the grace and knowledge of their Lord and Savior. As you can see, then, a certain degree of maturity is presupposed here. Yet, we should keep in mind that Scripture does not set forth adult maturity as a criterion for baptism. Apparently, Daniel, David and Samuel were among those God called into his service while they were still in their teen years. Polycarp, the disciple of John, was apparently baptized before he was a teen.
Some teenagers profess faith and then eventually fall away.
Perhaps their original profession was not genuine. That happens with adults, too, no matter whether they are 20 years old or 60 years old. Age alone is not what guarantees sincerity.