This article is mostly a rehash of how unprepared people are for retirement, but the following paragraph had an interesting statistic.

The first premise Ruffenach challenges is the belief that those who wish to will be able to work in retirement. Statistics suggest otherwise. While up to three-quarters of pre-retirees anticipate working beyond retirement age, only 12 percent of current retirees report being able to do so. What portion of that 12 percent work as Wal-Mart greeters or at some other minimum-wage job is not clear; but the lack of good-paying work combined with declining health (both physical and mental), prevent many retirees from pursuing employment.

When considering whether to continue working in retirement years most people focus on whether it would be worth it to them. They rarely consider the fact they may not be able to work. Obviously this varies a lot by profession and whether you are self employed, but it is worth keeping in mind.