What is the cause of this childish behaviour we see often? Am I wrong to say that, even if one is biologically an adult, that one's mind "is not" or is this just a side effect of teens being conditioned into thinking they are children. Because when one is treated as a kid, one does not have to account for one's responsibility.
It is a result of the Kaafir way of thinking, the Kaafir society, the Kaafir mindset. If a 19-year old adult believes that he is still a child and is not responsible for his actions, then he will start acting that way as well. However, if society were to treat everyone from the age of 15 the way Islaam says they should be treated, which is as adults (because they are), and held responsible for their words and their actions, they would not have the "childish" mindset.
If you look at the history of Islaam, there are so many examples of great feats and accomplishments taking place at the hands of teenagers. Why? Because as Muslims who follow Islaam, they knew and understood that they were "adults" and not "children". Take the example I gave in a previous post: Hadhrat Usaamah ibn Zayd رضي الله عنه, leading an army at the age of 15. Look at Muhammad al-Faatih of the Ottoman Khilaafah, who conquered Constantinople at the age of 17. Look at Muhammad ibn al-Qaasim: He conquered Sindh and Multan at the age of 17. He was executed by Sulaymaan ibn `Abdil Malik at the age of 19. So he only lived in this Dunyaa for 19 years, but yet he became so famous. In Pakistan, they have the holiday "Yowm-e-Baabul Islaam", which is celebrated in honour of Muhammad ibn al-Qaasim. The largest park in Sindh, "Baagh ibne Qaasim", is named after him. Port Qasim, second major port of Pakistan, is named after him. "Bin Qasim Town" in Karachi, Pakistan is named after him. "The Pakistan Naval Station Qasim", or "PNS Qasim", naval operations base for the Pakistan navy, is named after him. "Muhammad bin Qasim Road" in Karachi is named after him. "Muhammad bin Qasim Library" in Sujawal, Thatta is named after him.
He became that famous despite being killed at the age of 19. So, when a person has Imaan and when he realises the value of time, and that "adulthood" doesn't only start at 21, but rather, from the moment of Buloogh, he is able to achieve a lot.