This is a topic that can be argued for hours.
In South Africa, one became a major at age 21 until recently, and it's now 18. From the age of 18, one is allowed to drink, to drive (Not at the same time ) sign contracts...

But some people would appear dangerous behind the wheel of a car at age 18. Some people mature faster than others.

I can argue that my boyfriend, age 38, is still a kid in certain aspects of his thinking. Physically and intellectually he is very mature. (Ok, this is just a dig, but I'm sure you understand what I mean)

I'm 25, but have always considered myself to be older, and able to hold meaningful conversations with older people. But emotionally, in certain aspects, I am very immature.

I think it's hard to pin-point an exact age of when a child becomes and adult (from a development point of view) but from a legal point of view, it's written in black and white and just varies from country to country