From What Explains The Widening Gap In Retirement Ages By Education (footnotes omitted but bolding added):
Over the last three decades, the average retirement age has increased by about three years, to 64.6 for men and 62.3 for women. But this trend is not uniform across socioeconomic groups: for example, high school graduates are retiring just a bit later than in the 1990s, leading to a wide gap between them and college graduates. This brief reviews studies by the U.S. Social Security Administration’s Retirement Research Consortium (RRC) and others that examine several potential causes of the unequal increases in retirement ages by education....
The gains in the average retirement age since the 1990s have been driven almost solely by those with more education. Today, male college graduates retire three years later than high school graduates. While the story is more complicated for women because of the dramatic change in their labor force participation over the latter half of the 20th century, the overall message is similar: like men, the gap in average retirement ages by education has grown substantially. ...
Health is one of the most important factors contributing to the retirement decision. In fact, according to Munnell, Sanzenbacher, and Rutledge (2015), it is the single most important factor in earlier-than- planned retirement, even more than involuntary job loss. Health matters in two ways. First, workers who experience a health shock are more likely to retire early. Second, workers who were in poor health when setting their retirement expectations also tend to retire earlier than they had planned, suggesting that unhealthy workers are often too optimistic about their ability to work longer.
Blundell et al. (2017) also find that both health shocks and initial health conditions are key factors driving employment status at older ages. Not surprisingly, their results show that the less-educated are generally in worse health. Their study adds that health is an especially important driver of retirement decisions among less-educated individuals: the association between health and employment is strongest among high school dropouts, and becomes weaker with greater educational attainment...