1) The male becomes in charge of all duties such as prayer, fasting, zakaah, khoms and hajj, as soon as reaches puberty.
Puberty in the male takes place by one of three things:
A) Semen exit while sleeping. If this occurred at the age of 13, he reached puberty.
B) The coarse hair plant on the pubic. That is the area between the penis and the beginning of the abdomen, or the beard hair plant.
C) If none of the 2 signs took place, then puberty is reached by completing 15 lunar years.
2) Sharia laws are derived from Quran and narrations of Prophet and Ahlul Bayt(Peace be upon them all).
The law of (everything is pure unless we are certain it is najis) is derived from the instructions of Ahlul Bayt A.S.