What is the punishment for watching forbidden things and masturbating at a young age, and how can one repent from that?

Masturbation and watching lewd films and images are forbidden, and they are among the causes of masturbation. Whoever commits these acts must repent to Allah and resolve not to return to them, and increase in righteous deeds such as prayer, fasting, and charity; because "Indeed, good deeds do away with evil deeds." Furthermore, fasting reduces desire, and the remembrance of Allah and prayer protect one from indecencies and evils.