How to Deal with Adolescents in Islam
Question

I have a brother who is fifteen years old, and I would like you to write a message to him explaining the Islamic rulings having to do with this period of his life, with regard to puberty, purification, wet dreams and other matters pertaining to men, and everything having to do with that age, because I am not sure that he understands, and I am afraid that he is praying without having purified himself properly. I also would like some advice for him, and I hope that you will pray for him, asking Allah to send him good friends. May Allah reward you with the best reward.
Praise be to Allah.What does Islam say about adolescence?
We ask Allah, may He be Glorified and Exalted, to guide your brother and all the young Muslims, to show them the right way, open their hearts to faith, and make things easy for them.
The stage of adolescence is one of the most dangerous stages that a human being goes through in his entire life, as during this stage he undergoes physical, mental, emotional and sexual changes, and the devil is very keen to tempt him during this stage.
How to deal with adolescents in Islam
From studying what specialists and educators have said about this matter, we find that they offer the following advice:

Try hard to connect the young person with righteous friends, for making friends with righteous people will make him inclined towards righteousness, because friends influence a person. As the saying goes: Do not ask about a person; rather ask about his friends, for everyone follows the example of his friends.

So he should look for good friends who adhere to religious teachings, so that he can join them, worship Allah with them, attend prayers in congregation with them, and seek beneficial knowledge with them.
We ask Allah, may He be Exalted, to enable your brother to find righteous friends who can help and encourage him to do good.

Try hard to fill his time with activities that are beneficial and useful in both religious and worldly terms, and do not leave him with free time, for that is one of the greatest causes of trouble at this stage. Sign him up for any purposeful communal activities in your area, so that he can benefit and bring benefit to others.
Hasten to arrange his marriage, if possible, for the Prophet (blessings and peace of Allah be upon him) said: “O young men, whoever among you can afford it, let him get married, for it is more effective in lowering the gaze and guarding one’s chastity. And whoever cannot afford it, let him fast, for it will be a shield for him.” (Narrated by Al-Bukhari, 5066, and Muslim, 3464)
Offer a great deal of Du`a for him, asking Allah to bless him and guide him.
Encourage him to do acts of worship and to keep away from everything that is prohibited, dubious or disliked.
Do not let him use modern means of communication in improper ways; keep him away from them, except for what is essential. So, for example, do not let him use the Internet in an isolated part of the house, such as in a closed room. The way to do that is to put the computer in a shared area of the house, so that the boy cannot be alone, watching anything that he should not watch.

These were a few useful guidelines on this topic.
Fiqhi issues related to adolescence
With regard to religious duties that are required of the one who reaches puberty, we cannot mention them all in detail in a single answer. But we can highlight the most important of these duties, then refer the questioner to some books from which he can learn more about this matter. Hence we say:

Firstly: when the young person reaches puberty, he is obliged to adhere to all the rulings of Islam, by doing what is enjoined and refraining from what is forbidden. He must pray, fast Ramadan, give Zakah if he is one of those for whom it is obligatory, and go for Hajj if he is able for it.

As prayer is not valid without purification, the one who reaches the age of accountability must learn the rulings on prayer, because Allah, may He be Exalted, says (interpretation of the meaning): “when you rise to [perform] prayer, wash your faces and your forearms to the elbows and wipe over your heads and wash your feet to the ankles” [Al-Ma’idah 5:6].
It was narrated from Abu Hurayrah (may Allah be pleased with him) that the Prophet (blessings and peace of Allah be upon him) said: “The prayer of one who passes something is not accepted, until he does Wudu.” A man said: What does passing something mean, O Abu Hurayrah? He said: Breaking wind, silently or loudly. (Narrated by Al-Bukhari (135)
It was narrated that Ibn `Umar (may Allah be pleased with him) said: I heard the Messenger of Allah (blessings and peace of Allah be upon him) say: “No prayer is accepted without Wudu, and no charity is accepted from stolen war booty.” (Narrated by Muslim, 557).
A description of how to do Wudu is given in the answer to question no. 11497 . The rulings on doing Ghusl in the case of Janabah is given in the answers to questions no. 10790 and 2648 .
With regard to things that invalidate Wudu, please see questions no. 14321 and 11591 .
One of the signs of puberty is the growth of coarse hair in the pubic area and armpits. One of the Sunnahs of the Messenger of Allah (blessings and peace of Allah be upon him) concerning that is to pluck the armpit hair and shave the pubic hair. This is explained in questions no. 26266 , 9037 and 1177 .

Secondly: if something becomes obligatory upon an accountable person, it is obligatory for him to learn the rulings on it, because no one can follow a command unless he understands it and learns the rulings on it.

Al-Qarrafi (may Allah have mercy on him) said in Anwar Al-Buruq (2/148):
“Al-Ghazali stated in Ihya’ `Ulum Ad-Din, as did Ash-Shafi`i in Ar-Risalah, that there is consensus that it is not permissible for one who is accountable to do any action until he knows the rulings of Allah concerning it. Whoever wants to engage in buying and selling must learn the Islamic rulings concerning that; whoever wants to rent or hire anything must learn what Allah, may He be Exalted, has prescribed with regard to renting and hiring; whoever wants to lend or borrow anything must learn the rulings of Allah, may He be Exalted, concerning that; whoever wants to pray must learn the rulings of Allah, may He be Exalted, with regard to that prayer; and the same applies to purification and all other words and deeds. Whoever learns and acts in accordance with what he has learned has obeyed Allah, may He be Exalted, on two counts. Whoever does not learn and does not act has disobeyed Allah on two counts. Whoever learns but does not act in accordance with what he has learned has obeyed Allah, may He be Exalted, in one way and has disobeyed Him in one way.” 
Learning about religious duties is easy for the one for whom Allah makes it easy, so let each one strive to learn and acquire beneficial knowledge and act upon it, and let him ask his Lord for guidance.
And Allah knows best.