There are various types of maturity: emotional maturity and physical maturity are the two main types. Physical maturity is when you are physically older, whereas emotional maturity is when you think or act as an adult would; more sensibly and based on your knowledge.
You're not technically "immature" if you're not acting mature; you could be neither mature nor immature. Some examples of mature actions would be giving up when you know you're in the wrong, or not picking fights.
I wouldn't consider myself to be 100% mature, but I also wouldn't consider myself to be completely immature either. I'm not the sharpest crayon in the box, but I also wouldn't think of myself as being dense and mindless.
I think that no one person is absent of immaturity. Some people may claim that they are mature in every aspect of their being, but knowing that you yourself and everybody around you knows that's not true is an example of immaturity.