Puberty

Puberty is the process of physical changes through which a child's body matures into an adult body capable of sexual reproduction.

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Sayyed Muhammad Husaini Ragheb, Sayyed Muhammad Husaini Ragheb has a BA in Law from Guilan University, Iran and has also undertaken Hawzah studies in Qom. He used to be a Cultural Affairs director of Ethics Group of Al-Mustafa... Answered 2 years ago

What is regarded as Mustahab is to remove that hair with whatever means is possible. So as soon as they start growing it, it can be recommended to them.
Just one thing to mention is that some people might think that if it grows long the prayers will be nullified, which is NOT a correct understanding.
Also it is NOT allowed to show one's private parts to those who remove the hair and unfortunately these days it's getting more and more.

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Zaid Alsalami, Shaykh Dr Zaid Alsalami is an Iraqi born scholar, raised in Australia. He obtained a BA from Al-Mustafa University, Qom, and an MA from the Islamic College in London. He also obtained a PhD from... Answered 5 years ago

Bismihi ta'ala

There is shar'i matter of Islamic maturity, which is bulugh, and as you know the boy needs to experience one of three things for him to become baligh:

1. Having a wetdream.

2. Growth of coarse hair around the private parts.

3. Completion of 15 lunar years.

If one is not close to this boy, or does not directly ask him, they would not know if the boy is "baligh". 

That is why there is something else that is very important, and that is the age of discernment (sin al-tamyiz).

This basically means that he has reached a certain age where he is able to discern between right and wrong, good and bad, and he understands his surroundings. 

When a boy reaches such a stage, he must be taught and also treated like as if he is baligh. He cannot show his private parts, nor can anyone else see his private parts. His should pray, fast, and do other wajibat, and stay away from muharramaat. Hijab must be observed in his presence, and everything else related to what a mukallaf should do.

As for what age does a boy become discerning, in general, this is defined individually by noticing his conduct, mental capacity and behaviour. 

And Allah knows best. 

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Zaid Alsalami, Shaykh Dr Zaid Alsalami is an Iraqi born scholar, raised in Australia. He obtained a BA from Al-Mustafa University, Qom, and an MA from the Islamic College in London. He also obtained a PhD from... Answered 6 years ago

Bismihi ta'ala 

According to the mainstream view of Islamic jurists, a female reaches the age of religious maturity, or bulugh and becomes religious duty-bound (mukallafah) at the completion of nine lunar years.

This is irrespective of whether she has reached physical puberty, or not. 

The evidence for this are authentic traditions from Ahlul Bayt (a.s.). As Muslims we believe that our religion has taken into consideration all aspects of a human being, whether it be physical, social, mental or spiritual. It is for this reason that we must adhere to the legislative rulings conveyed to us through our jurists, as they have deduced these rulings through the primary sources of Islam. 

I would like to further add that this is indeed an honourable privilege for females, as it means Allah ta'ala has blessed females with something quicker than males. There are two conditions for takleef becoming obligatory on a believer: 'Aql, which means rational comprehension, and qudrah, or ability.

A female will surpass a male in acts of worship and obligations and devotion to the Almighty, which is also another valuable privilege. She becomes responsible, accountable and independent in the religious side of her life. She will develop quicker than a male, and mature in all ways faster than a male. 

Furthermore, we must not mix between puberty, and entering into the age of takleef.

And Allah knows best.