That people were generally grown up and ready to face the world when they were 18-20. There might be rough edges or blind spots, but that'll get ironed out with a little bit of experience.
My first week at college quickly disabused me of that notion.
People didn't know leaving food out would cause it to spoil, that pizza boxes rot and attract vermin, didn't know how to do laundry, clean up after themselves, that getting enough sleep was necessary to function properly and letting other people sleep was the courteous and polite thing to do... Basic life skill stuff. The minute mom and dad weren't around to do everything, they had no idea how to do anything. And this is before we even get into emotional intelligence...
And these were people who were admitted to one of the best universities in the country, if not the world.