Why would society not consider a 20 year old to be a grown adult?

we become an adult when we are totally independent from our parents financially and in every aspect of our lives and our parents become like old friends.

In reality most of the world considers 18 to be the legal age of adulthood. I was brought up in the UK when the age of majority was still 21. I went for a polio injection when I was 19, and they refused to give it to me without my parents consent!! I pointed out (in those days) i could fight and die in the army, or get hanged for murder, but could't get an injection.

The world is a crazy place.

There is a lot of clinical evidence that, whereas the female brain is fully mature around 20, the male brain does not develop full judgement capacity until around 25. Hence the crass stupidity of 20 year old males.

from my personal experience I think bein a an adult mentally starts about 26-27

Its that perfect unblemished skin, the innocence, the tiny ears. All that stuff makes you still look like a kid, although mentally you are just coming into your own.