In a “young adult” book the main characters are typically between the ages of fifteen and nineteen. If the age is lower it might be categorized as a middle reader. In adult fiction the main characters are typically age 20 or above and can be any age.
Another main distinction between young adult and adult fiction is the voice used and the issues and themes that relate to the young adult experience.
In young adult there is more immediacy and in adult fiction, there is more reflection.
So in other words, whatever you enjoy is what you should read. We simply try to make it easier for your to find your next book by conforming to industry standards.