Depends on the person and their situation. It can either be not really adulthood at all, or some of the most challenging years you'll ever have in your adult life.
Some students still live with their parents, have their parents pay for their entire tuition, not need to work part time at all, and generally don't have to worry about that much more compared to high school, with the exception of increased workload and a slew of new experiences to make sense of.
Others have no support or perhaps even contact with their parents altogether, are forced to take out loans which can range from modest to astronomical depending on the country (or juggle full time work and school simultaneously if the parents refuse to
co-sign), at least work enough hours part-time for it to eat into the energy that "should" go to coursework, have tremendous difficulty securing housing without a full time income or a
co-signer, etc.
Pretty much everyone falls somewhere along the spectrum between the two extremes. I was very much skewed towards the latter extreme. My full time working life is so much "easier" compared to when I was a poor student caught in legal limbo all alone.