Insulin resistance is considered the precursor of impaired glucose tolerance--it's the point at which it take significantly greater pancreatic release of insulin to drive glucose into the cells, but the body can still make enough to keep serum glucose from rising to impaired levels.

In time, though, unless they make lifestyle changes, many can no longer produce enough insulin to keep their glucose below such levels.

I suspect that a great many people (possibly even a majority) past 40-45 who eat a standard Western diet rich in simple carbohydrates, and who do not exercise vigorously, have insulin resistance to greater or lesser degree. Our metabolisms evolved under a feast or famine mentality, making it natural for many of us to store fat under conditions of plenty. Unfortunately, nowadays these are generally not followed by hard exercise or periods of scarcity, and this accumulation of fat over times make insulin work less well.