I dont but maybe because when you come to the age of 18 you have already experience a lot of stuff so you are now more capable of making decisions like an adult also probably because most puberty ends here and that will mean not much of childish behaviour anymore because hormones stops being abnormal idk
Sorry, I tend to beat around the bush. Σ(  ̄□ ̄||)
In the middle ages, the mortality rates on children was extremely high. Which is why in most cultures, children had to mature quickly, in some cases the age of adulthood was as young as 10... However, in Europe, still in the middle ages, the age of adulthood became 18. That's because war between neighbooring countries was extremely common. Their armor was extremely heavy and could only be worn by men 18 and up without damaging their bodies.
I know the answer and am willing to share! I just want to know what you all come up with, WITHOUT using Google. (๑•ㅂ•)و✧
I look forward to your ideas!!