What would you say to a young teenager who committed fornication and who came out for altar call?
There is no "altar call" in the Bible. But if a young person wished to become a Christian and was willing to give up his sins, then that is just what becoming a Christian is all about.
"Do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived. Neither fornicators, nor idolaters, nor adulterers, nor homosexuals, nor sodomites, nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners will inherit the kingdom of God. And such were some of you. But you were washed, but you were sanctified, but you were justified in the name of the Lord Jesus and by the Spirit of our God" (I Corinthians 6:9-11).
The article What Must I Do to be Saved? covers the requirements for becoming a Christian. It is at the point of baptism that sins from the time before becoming a Christian are washed away and a new life is begun.
"And now why are you waiting? Arise and be baptized, and wash away your sins, calling on the name of the Lord" (Acts 22:16).
"Or do you not know that as many of us as were baptized into Christ Jesus were baptized into His death? Therefore we were buried with Him through baptism into death, that just as Christ was raised from the dead by the glory of the Father, even so we also should walk in newness of life. For if we have been united together in the likeness of His death, certainly we also shall be in the likeness of His resurrection, knowing this, that our old man was crucified with Him, that the body of sin might be done away with, that we should no longer be slaves of sin. For he who has died has been freed from sin" (Romans 6:3-7).
"In Him you were also circumcised with the circumcision made without hands, by putting off the body of the sins of the flesh, by the circumcision of Christ, buried with Him in baptism, in which you also were raised with Him through faith in the working of God, who raised Him from the dead. And you, being dead in your trespasses and the uncircumcision of your flesh, He has made alive together with Him, having forgiven you all trespasses" (Colossians 2:11-13).
"He who believes and is baptized will be saved; but he who does not believe will be condemned" (Mark 16:16).
"Then Peter said to them, "Repent, and let every one of you be baptized in the name of Jesus Christ for the remission of sins; and you shall receive the gift of the Holy Spirit"" (Acts 2:38).