Oh my, you and I had some similar frustrations. I am 39 and let me tell you what I wish I had known. Most adults are dumber than you. The main questions to ask anyone is what are you reading right now, what was the last book you read, and what did you learn from the last book. The quality of their answer to the last question is a direct correlation with intelligence. Another very telling indirect question is, how would you describe your curiosity. Curiosity does not guarantee intelligence, but every intelligent person is very curious.
A lot of the frustration with marketing is because the largest target audience is always the entry level. Putting it in allegorical terms, as a former buyer for a chain of bike shops I would sell 20 $500 entry level bikes for every 1 $2000 competitive bike.
The lowest level is always the main target audience. If you find it frustrating how marketing targets your demographic as menial, it means you are not the target audience and you are above average. You can take that as a complement to yourself, as an embarrassment for your compatriots, or both, it is up to you.
Life’s experiences will determine if or when you ever feel “adult.” A lot of that is from having kids and the difficulties involved. Most people never really feel adult. There is no moment of transition. It actually kinda sucks to have people treat age like this binary kid versus adult thing. Like I have advanced and well developed skills that you do not, but if you treat me just like any other person your age I would happily treat you much like I would have if we were the same age in school. Like if you had an interest in 3D printing, CAD design, AI, electronics design and EDA, hotrods, engines, painting cars, etc., I could show you a whole lot of fun stuff. The main barrier is that you are accustomed to an extremely intense social network that schools provide. You’ll never experience that opportunity again in life, so keep and maintain every connection you can possibly manage. As you age, life gets more and more lonely for most people. This is the hard thing to overcome in the reverse. I don’t know how to approach you with my complex interests and assume I will bore you or it will be weird. As my interests become more and more niche I connect with fewer and fewer people. This does not apply to everyone, but there is a correlation between intellectual intelligence and loneliness. I don’t mean to discount the value of emotional intelligence. That is just an area with which I am not particularly familiar.