When does namaz become farz on a muslim boy, when he turns to be mature regardless of age?
Or namaz will be farz when boy is 15 years old, when he completed 14 years of his life?

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Puberty in males could be confirmed if one of the four signs was present:
1. First: Completion of fifteen lunar calendar years of age (equal to about 14 years and seven months and fifteen days of the solar calendar).
2. Second: Ejaculation through sexual intercourse, or seminal discharge while awake or asleep.
3.Third: The presence of pubic hair, of the rough type.
4. Fourth: The presence of hair on the face and above the lips.
Note: The presence of under arm and chest hair and the deepening of the voice are not the signs of puberty in Islamic law.
He should begin performing his religious duties when he attains the age of puberty.