I'd guess it's mostly genes. And not that it's pre-determined by genes and there's nothing you can do about it but for some people it simply just is easier to stay lean with little effort compared to someone else. I don't eat particularly "healthy" but I eat a lot at once and then I might go the rest of the day without eating. I regularly skip meals aswell. When you add in the regular gym workouts I've been doing for over a decade plus weekly mountain biking trips I guess it's no wonder I've still got that "swimmer body" in my thirties. Also I never really "got into shape" either - I never just got out of it.