How does a person become an adult?

Nowadays, to use a word"Adult" needs to be careful. It means almost "things which law forbids a child ".I wonder when it became so.
Formally It meant having an career and preparing for marriage. but now nobody seems to use it as  such meaning. I come not to hear "You are a grown-up,aren't you".Nobody can't know What is to be an adult.
 well,I try to define it as follows.
An adult wash his hands after using bathroom.
An adult don't pick up magazines  remained on the road.
 An adult don't read a comic book.
An adult mustn't say to a bride "Congratulations !"at the wedding party.
An adult woman mustn't be a dog in the manger to more pretty young girl than herself.
An adult blogger posts comments after he read the blog exactly. 
An adult mustn't take it on others when his favorite baseball team lose a game.
An adult subordinate can put up with a boring preaching of his boss.
An adult dog don't cling to a lady.
I may be told by someone"You are still child."