I'm putting together a historical fiction syllabus right now and I know that one question I am going to get is this: what defines historical fiction? It's not simple. Some argue before living memory, say 100 years ago (which cuts off my own novel, not to mention much of E.L. Doctorow's work), or others believe it's two generations ago (which I can't really define). I tend to define it in looser terms: stories where the historical setting is at the forefront of the narrative.
For instance, I just finished Rachel Kushner's The Flamethrowers, which is set in late 1970s New York and Italy. The fact that it's forty years ago is essential to the narrative. In addition, while it feels somewhat like our current world, it is definitely different in fundamental ways. It's in the past, one that is strikingly different than our own. While it's in the "living memory" of many readers, it's specifically about a different time than the contemporary reader is living in now.
I was also thinking of this because I just began reading Marlon James monumental A Brief History of Seven Killings. It's set in Jamaica and the U.S., spanning over thirty years (1959-1991). Is it historical fiction? I lived through some of these years (as did James who is somewhat older), but much of it takes places before my memory. Most would say it's not: it's too recent. Yet, it's a story that could not take place in the modern age, but speaks to a modern audience.
In the end, defining genres is sort of pointless. It doesn't particularly matter and the stakes in doing so often result in the sort petty fights that take up much of our current literary culture.