Continuing with one of the ideas from the post below, one landmark of growing up is changing your mind about some things that you hated from your childhood and adolescence. Typically we think of young people as more open-minded, and the personality trait Openness to Experience does peak in the late teens and early 20s. However, this is more of an openness to new or different stuff. I'm talking about being open-minded enough to admit that you misjudged something good as bad -- a much tougher error to admit than the other way around.
When you're a teenager or young adult, you're too socially desperate to question the worth of your behavior and your culture -- you just have to shut up and go with it, or else you won't fit in with your clique during a time when you are unable to go it alone. Once you're more socially independent, which happens sometime during your mid-to-late 20s, you can relax your narrow-minded devotion to your group's products and practices. You're even free to borrow things from a rival faction and not suffer as much: ostracism is less powerful when social relations are less tribalistic.
How do we know that tribalism declines after the early 20s? Simple: look at how conspicuous your group membership badges become. Even adults belong to groups and signal their affinity, but the intensity of the signal is lower and the noise around it is greater. For one thing, unlike teenagers, adults don't wear clothing with logos, or with names, pictures, and other icons of their favorite entertainers. Their use of slang is a lot less frequent, and the turnover rate is much lower. Some professionals have jargon, but it's nothing like the slang of teenagers, which obviously functions as a set of shibboleths.
As a concrete example, consider all of the pop music groups who disgusted you as a young person. (Which pop music groups we follow is one of the main ways that we express our tribal membership.) It's possible that all of your judgments at the time were correct, but it's not very likely. Now, some of them you'll grow to like just because your tastes involuntarily change with age, like preferring the more bitter espresso to Frappuccinos or the more pungent Roquefort to cream cheese. But these don't require questioning your earlier assessments.
Take a group that was pretty popular when you were a teenager, but one who your tribe was steadfastly against. If you had even considered listening to that group, your peers would've threatened you with excommunication -- "we don't listen to them." Once you can more safely tell those people to fuck off if they don't approve of your musical tastes, you start to re-examine some of your earlier judgments and find many of them to have been wrong. You may feel that they were necessary and rational in the context of surviving tribalistic adolescence, but still they were unfair. All of a sudden, your mind opens up to wholly uncharted waters of the cultural oceans.
At the same time, you don't overturn all of your previous decisions. Motley Crue really did stink -- no mistake there. But if you don't uncover at least a couple of faulty convictions when you rummage through the volumes of your life experience, you haven't matured yet. To see that their underlying merit is what gets a sentence overturned, many people who re-examine the same collection of cases will independently arrive at the same conclusions -- that one was dealt with fairly, but this one shouldn't have been punished. Unlike Motley Crue, Guns N Roses are much more likely to have their credibility re-established, quite simply because they were better.
To measure how much more open your mind has become, we just ask how you respond to the cases that would sting the most to admit you were wrong -- namely, where their worth was so great and yet where you spilled the most blood. There's no one perfect example of this, but disco sure comes pretty close. At least within the realm of pop music, few other groups can match Chic on musical innovation, virtuosity, complexity, and breadth of emotional range. If the most sophisticated evaluation you can make is that "disco sucks" or that only queers like dance music, you still have a lot of growing up to do.