Is the Hadd punishment carried out on one who reviled Allah's Prophet Jesus (peace be upon him) while being fourteen and a half years old and showing signs of puberty? And does the Hadd punishment lapse for him upon repentance, given that there is a fatwa stating that it is wrong to kill someone under the age of fifteen?

You have committed a great sin by insulting one of Allah Almighty's prophets, which is an act of disbelief (kufr) and apostasy from the religion of Islam. However, whoever repents with sincere repentance, their repentance is accepted, and Allah erases the sin from them, for "the one who repents from sin is like one who has no sin." Punishments (hudud) and discretionary penalties (ta'zirat) are only to be carried out by the ruler (wali al-amr). So, if your action has not reached the authorities, you must conceal yourself with Allah's concealment and repent privately between you and Allah. You should not dwell on this matter, but instead strive in obedience to Allah and increase in good deeds, and recognize the status and standing of the prophets. Be cautious of debating Christians and others without sufficient knowledge to refute misconceptions.