I read almost exclusively YA as a teen. I probably started reading YA in the late 70s, when I entered junior high school, and read it at least until 1984-85, which was my first year of college. So, it was definitely out there. Some authors I remember reading were Judy Blume, Paul Zindel, Paula Danziger, Ellen Conford, Julian F. Thompson, Richard Peck, Katherine Paterson, Hila Colman, Marilyn Sachs, Lois Duncan, Madeleine L'Engle, Zibby Oneal, Norma Klein, Norma Fox Mazer, M.E. Kerr, S.E. Hinton, Francine Pascal (*before* Sweet Valley High, which I never read), and also some of the Sweet Dreams and Silhouette series romances, but I preferred the standalone books. I remember seeing Fast Times at Ridgemont High as a "book based on a true story" before the movie was ever made. I also remember one YA novel by science fiction writer Andre Norton.
A few of those went into what would be considered midgrade territory now (protagonists around 13 years old), but that was considered YA at the time. I still have some old paperbacks for that age group with "young adult" on the spine. But some were definitely at the upper end of YA and edgy, with one book I remember being labeled YA but having a 19-year-old protagonist (and sex, drugs, and suicide...). I went to the bookstore probably once a week to buy a new book or two, usually Waldenbooks or B. Dalton at the mall, or a local Texas chain then called Century Books, and I also remember going to Half-Price Books and other used bookstores, and also hanging out in the library a lot... and never finding what I wanted in the "grown-up" section. In the children's/YA section, I would pull book after book off the shelves to read the jacket copy or opening pages, and nearly always find a few that piqued my interest, but doing the same in the adult section always seemed frustrating. The adult novels usually had plots I couldn't relate to or just didn't care about, and the pacing was usually too slow for my tastes.
I am embarrassed how little adult fiction I read, or in fact have ever read! When I got to college and felt too old for YA, I only read a few novels a year (including some that had kid or young adult main characters, anyway!), because I never found a lot of adult books that appealed to me in the way midgrade and YA books had, until I was about 24 or 25 and realized I'd always wanted to write YA, so I gave myself permission to read YA again and write it.