When you think of maturity, often you think of the phrase, "With age comes wisdom and maturity." While in many people this is very true, this is not the dead ringer truth across all humankind. Maturity is not a state of being. You are not simply a mature person because you've been alive longer; there are plenty of 60-year-old's who are much less 'mature' than some 22-year-old's that I know. Similarly, being mature doesn't apply to a state of being just because you've reached a certain age, like 18 or 21. These ages are simply to indicate whether or not you are old enough to drink, and how you should be tried in a court of law. It may imply that you are capable of responsibility, but it does not imply that you are mature. This misconception leads us to thinking that 'mature' and 'responsible' are the same. Yes, you can be both mature and responsible simultaneously, but they are not synonymous. Responsibility shows through doing chores on your own; paying bills on your own; working on your own; getting your homework done; practicing for music class or a sport; really, doing what you're supposed to do in order to continue functioning properly in your own life and in society. Maturity is determined not by what's required of you, but by which decisions you make. Yes you can be responsible by going to your work; do you choose to do your job poorly (i.e. stand around talking; cut corners on responsibility; be dishonest in your clock-in/clock-out time; etc.)? Sure, you can learn everything there is to know about "the adult world" but are your motives correct? Yes, you can think for yourself, but are you wisely considering advice from those around you? And how are you acting and reacting in any given situation? Anything you do in life can surely be labeled as responsible, but just because you are responsible for these actions does not mean that you are mature. Maturity is not a state of being that you have as you get older; it's in your decisions, actions, and reactions, and reactions to reactions. Responsibility can be handed to you pretty easily; or you can assume responsibility on your own. But maturity does not come as easily as you think it does, and it's not one decision that makes you 'mature'; it's every decision that makes you mature - even the bad ones that you have to learn from.
Maturity is not a state of being. My friend was more right than I was in realizing that the reason she thought she was mature back then was because she couldn't see all of her flaws and struggles; recognizing where your flaws are - and admitting to them - is a good starting point to maturity. Denying or ignoring your short-comings hinders your very growth towards the maturity you desire. Be humble; be merciful and graceful - to yourself and to others; own up to your mistakes and work diligently so that you are blameless. Remember that maturity is not necessarily a state of being, but it's rather that you are mindful of your being entirely and what you choose to do based on that mindfulness.