All I can do is give you an answer based on my own experience, so here goes.
Immature INFJ
Insecure, shy, not understanding why people don't get you and being bothered by that fact, feeling like an outcast, sitting along the sidelines in a stagnant existance, ignoring or not even knowing what your intuition is, a desire to "be like everyone else", not liking yourself because you are different, you pull away from situations easier, worry wart, submissive in most situations. Fear holds you back.
Mature INFJ
Self acceptance, knowing you are unique and appreciating that fact, realizing not everyone will understand you and that's ok, you listen to your gut and realize intuition is meant to guide you, you don't want to be like everyone else, you have an interest in watching and studying people and why they do what they do, don't sweat the small stuff, find power in submissiveness yet you aren't afraid to speak your mind. Fear drives you to create change.
Of course these are just a few things but maturity is a tricky thing. We are always growing, always learning, always changing. Maturity hinges on life experience and understanding - we all experience those things at different rates. Keep an open mind and grow.