This attitude bothers me. As a 14 year old I was pretty sure where I was making good and bad decisions, when I could do something properly vs when I didn't have enough information or experience, etc. But adults kept telling me I wasn't old enough, I lacked maturity, and so on.
So a few years later I went to college. And now I wasn't some high school kid, but I still wasn't an adult. Adults told me I lacked maturity, I wasn't old enough, my brain was still developing, etc.
Then I graduated college and I learned the secret: there's no difference and adulthood as a concept is all a scam. The *person* is what matters. Every kid attaches a certain amount of weight to things "adults" say because that title carries weight. But I learned: some adults are smart, and some are basically that dumb kid in high school you got sick of listening to. Just like in high school, some kids were smart, and some were basically dumb.
My personal theory is people are basically who they're going to be by age 15-16, and don't change much from that point on. Society changes around them.
Why do people assume you - to use your words, "know the music"? You're presumed to because... why? Something you did? Something you've demonstrated? No, when you walk into a room, people observe that you've over this arbitrary age threshold and figure you're basically mature. Sure, you could dispel them of that presumption, but some mature teenager has to start from the presumption that they're immature. Some might call this "privilege" (not I, but it's the same concept).
Ultimately this wouldn't matter much except by devaluing teenagers, we overvalue adults and do dumb things because we overestimate the average person. If we looked at the people around us and though of our high-school classmates I think we'd have a much more realistic outlook. "Immature behavior" is really just standard human weakness by another name, but easier to dismiss and harder to address since you assume people just grow out of it, when really they just lose the excuse.
And for the record, I've got a full-time job (going on 5 years and 2 promotions), an apartment, a 401(k), stocks and other assets, savings, life insurance, I do my taxes, etc. I'm 27 and I haven't changed a bit since 16 - people just give me more credit. To be clear I don't say I'm "wise" - I don't have that kind of experience - but I'm as mature as I'm ever going to be.