It is important to know our self? Do you know your self well?

I hear from some songs about knowing one self and some other people do say that in giving advice in some tv programs where there is some doctor who will advice one guess on knowing one self is a key to recovery.

But my question, at what age have you think that you are really know your self well enough. is it that important to know our self and what advantages there is in doing so. for me to be happy we should know who we are and what are the things that makes us happy and what we really want to be and do in life. only by discovering that we are able to really happy and successful in what we want to achieve in life.