I read somewhere that it was a third of guests to WDW are adults travelling without children. I can't remember where I read it, and have no idea how accurate it is, but we read it whilst we were dithering about whether to book our first trip or not. It tipped us over the edge to book it, as we felt like we wouldn't be the odd ones out! We have to travel during school holidays (I work in a school), but we've never felt out of place... despite getting grief from "friends" of my husband!