I suspect you don’t quite get what the writer is trying to say. Maybe you have limited experience with urban settings?
Because there’s a large aspect of social / music / food /etc in urban settings that is effectively 21+. It can be a real issue, especially if you’re evening-chronotype and you’re “underage”. It limits what you can do, who you can do it with, and even what hours you can go to certain places (restaurants that card after a certain hour, for example).
That infantilizing is part of why we have a generation of dysfunctional millennials who were taught they were children well into adulthood. Not everybody follows the “I’m 18, I’m still a child!” mentality, thankfully, but for people living adult lives in the 18–21 range, it makes things complicated (in many ways, including alcohol).
I once dated a very sophisticated 20-year old when I was ~30, and it was pain to go many places. Never mind that she was more mature (and more cosmopolitan) than many people 10 years her senior. Out for a nice dinner, I’d order something to drink, and the waiters would give her grief even though she wasn’t drinking or ordering (because wine with dinner is clearly a disorderly act!). Much less being able to join me and my friends at a bar for a social outing… Alas, America is so backwards sometimes.