Speaking from my own experience: I just started college with the age of 18 and I am actually below the average age of freshman in my country - mainly since many feel the need to put in a gap year. That hasn't been my goal since I didn't feel that I would truly embrace spending one year more or less doing nothing. Like many other radiators have already said, it's most definitely a mentality thing & college is honestly the one place which shows me how immature certain people are, and how far they are away from reaching adulthood. We have spoiled brats reaching from the age of 17 all the way until 25.
In my opinion adulthood requires the understanding that one is solely responsible for their own life and the direction they take it. Being enrolled at a college and adhering to a given budget from some rich parents does not equal being "responsible with finances" nor any sort of self-reliance. When one is unable to rely on themselves for the entirety of their life, I can hardly imagine how they call themselves an adult.
Furthermore - Part of growing up or becoming an adult requires realising for yourself what it is that you wish for your life to turn out to be. This is a thing that I am currently struggling with since I have always had the academic route implemented within me (childhood influence and "rationality") yet few months into college and I have 0 desire to continue down this academic path. Standing up for yourself and deciding to risk it all even though your entire family may be entirely against it - that's a certain type of courage that an adult has to have. This is why I wouldn't characterise myself to be an adult since I am far away from seeing myself capable of doing that, but anyway ...
I have a friend who was unlucky enough to see both of his parents away at the age of 18 with little to no money left in their household as they were scraping by. He was able to get a job and care for his smaller brother for the next years and has now gotten a job with a 6 figure salary. I think there is hardly a better example for someone who truly underwent the "true transition" to adulthood.