I've actually seen both sides of the maturity line in the sense that there were times in my teens when I acted very mature (at the age of 14, people on one forum thought I was in my mid-twenties), and yet there were other times when I was called out quite sternly for certain things I had said. Over the years and through discussions with—or observations of—various people, I've gained some perspective on the subject and can say with some level of certainty that people do not simply 'gain 1 point in maturity!' every once in a while while aging. People are multifaceted, and each facet matures individually. Some otherwise mature individuals are terrible at handling their finances; some are bad at dealing with annoying people; some can't keep a secret; some swear all the time because they think it's funny; et cetera… (I just geeked out after typing that and noticing that the Latin and ellipsis are redundant <--- a good example of minor immaturity)
Immaturity is part of life (sometimes a way of life

) and actually a very good one. It's burned into our psyches for the same reason learning curves are in games. Too much information can ruin the experience and cause you to burn out. We have a tendency to give ourselves 'time off' every once in a while and being purposefully immature in one area of your life while holding yourself to a high standard in others is essentially giving yourself a break.
People can attain 'enough' maturity at almost any age. I've seen very mature people below the age of 12, for instance. (GeekNote: In some cultures, people were considered adults at the age of 13 :EndGeekNote) However, most modern kids are much slower to progress, and
need to be given the time to do it gently, without pushing all of the 'mature' content in popular culture on them all at once. I know for a fact that seeing several PG-13 and a couple R-rated movies affected my early growth negatively, though I won't go into detail.
Anyway, I know I'm not completely mature yet; I still lose my temper frequently (I absolutely LOATHE smirks, don't use it against me >:[ ) when dealing with young children, though I have improved in that regard.
Bah, now I'm just rambling. It's time for bed.

LAST NOTE: 'large group of the Earth's population #1' are annoyed with 'kiddies' because 'kiddies' ask questions that 'lgotE'sp#1' believes the answer to is obvious, and they dislike the idea of wasting their time explaining it again, when they think that next week's or next month's or next year's batch of 'kiddies' will ask the same thing, repeatedly, for the rest of their lives. It's no coincidence that 'lgotE'sp#1' is the group of people that tends to make FAQs on fora, then put links in their signatures pointing to them. The 'kiddies' age has nothing to do with it. It's just that, as another poster alluded to, "Get off my lawn! It's my lawn and I don't have to explain why you shoudn't run through it!"