Julia, can you perhaps remember the genre of the book at all? Was it Gothic fiction, horror, non-fiction, adult fiction, young adult, children's fiction, etc? That might help narrow it down a bit, or perhaps any more details at all?
I found one titled "The Girl Who Owned a City" by O.T. Nelson, published 1977. It's about a virus that wipes out anyone over the age of twelve. But that's all I could come up with without more info.