What is the ruling on masturbation for a non-pubescent person, and how does one repent from it?

Masturbation is forbidden even before puberty. The age of puberty is 15 years, or upon the appearance of its signs. Sin is only incurred by one who has reached puberty, but accustoming oneself to masturbation is harmful for the future and leads to missing out on good. Abandoning this habit is in compliance with Allah's command, and it falls under the description of the people of Paradise who guard their private parts. The Prophet (peace be upon him) guaranteed Paradise to whoever guards his private parts. He (peace be upon him) said: "And whoever seeks chastity, Allah will make him chaste."