Children have no corner on imagination, nor can you limit fantasy to the experience of adolescents. Kids are perfectly capable of reading about adults, and will do so, if the adults do anything interesting — as they do in science fiction, for instance. (Most kids find novels about adults in dysfunctional families with dreary sex lives in the suburbs uninteresting, and by God, they’re right; but that’s another topic.) Also, adults will of course read about kids, if the kids are doing anything interesting — You there, Huck? — The whole idea of YA as a literature apart is shortsighted and arbitrary. But it’s marketing, so it’s a sacred cow. Milk it, and question not.
I’m with Le Guin and her imagined kids – I read very little of the kind of contemporary literary fiction that wins awards, gets reviewed in the right papers, and makes the NY Times bestseller lists because I find it tedious. It’s good to know that it’s not just me.