If older workers are wondering why it's so hard to save for retirement—particularly as they age—the answer may lie at least partly in their paychecks.

The blog for the Center for Retirement Research at Boston College pointed out a study that tracked the inflation-adjusted earnings of some 5 million workers over their lifetimes. The study, from the Michigan Retirement Research Center at the University of Michigan, not only found that "earnings risk varies significantly across the population," but uncovered four trends within that risk that have to be disturbing for almost anyone who ever wants to retire.

So what are these disturbing trends? First, while the size of income risk falls between the ages of 25 and 50, it increases from 50 on. Second, the older people get, the more likely it is that income shocks will be either very small or very large—no middle ground here. Third, the older people get, relative to the average change in income, a large fall is more likely than a large rise. And fourth, which perhaps will come as no surprise, people whose lifetime earnings are in the top 5 percent will have a growth rate between the ages of 25 to 55 that's 10 times larger than people who only have average lifetime earnings.