What is being an adult?

What is being an adult?
Some people say ‘the Big Three': taking responsibility for yourself, making independent decisions, and becoming financially independent,"
For me myself, I want to be proud of being who you are.
because initially I'm not confident with myself, I want to achieve stability with a career, or a past that I can be proud of that I have passed the obstacles of life.
But recently, my boss comments me than do not be a helper but be PIC, if you want quickly assigned the job. He is right, I fully aware of that. 
Becoming adults is become able to move on solo act by their own, to have their own thoughts and solutions of every problem they’re face.
That was ideal.
Maybe this time I'm still on the way maturity, even though the age of 28 years.