It depends on what you consider mature.
To quote from the Wikipedia link you provided:
"Maturity is a psychological term used to indicate how a person responds to the circumstances or environment in an appropriate manner. This response is generally learned rather than instinctive, and is not determined by one's age. Maturity also encompasses being aware of the correct time and place to behave and knowing when to act appropriately, according to the situation and the culture of the society one lives in."
Personally, I was under the impression I'd matured quickly. As a child, I was called a "little old man" or told that I'd "been here before" and adults told me that talking to me felt like talking to an adult. In adolescence, I was told that I was immature and that opinion has stuck. Even though I'm less ignorant, judgmental, petty, inconsistent, dishonest and prone to folly than other people of my age group, I'm immature because I haven't picked up the appropriate behaviour. The appropriate behaviour seems to be a lack of concern for others, placid indifference and flaky morals, with a noticeable lack of idealistic ambition.