A recent publication by the Institute of Economic Affairs (PDF format) looks at studies that suggest retirement leads to worse long-term health and shorter remaining life expectancy. You'll find the meaningful discussion on how researchers went about trying to identify cause and effect in the PDF rather than the press article quoted below: does the data actually show that retirement causes worsening health versus a tendency for people with worsening health to retire, for example?
A study out of the U.K. suggests that while it may provide an initial sense of relief and well-being, over the long-term, retirement is bad for your health, increasing the likelihood of developing depression and at least one physical illness. The study's author [analyzed] data from a survey of 11 European countries that sampled 7,000 to 9,000 people between the ages of 50 and 70 using two separate methodologies. He found that retirement had a "consistent negative impact" on physical health that worsens as the number of years spent in retirement increase.
[The author] also analyzed past studies on the subject of retirement and health and found that their results were mixed, with some finding a positive impact and others a negative or neutral one. The researcher attributes these varied results largely to a failure to distinguish short-term effects from long-term ones and to take the length of retirement into account. In the short term, retirees may experience a boost to health, he says, but this is outweighed by the negative impacts that manifest over the medium and long term.
[The author] acknowledges that there are many variables in any one individual's retirement that can often have contradictory effects on physical and mental health. Retirement can decrease work-related pressures and stress, for example, but it can also cut retirees off from the social networks they formed at work and lead to greater isolation, which can negatively affect health. By contrast, it can lead to more leisure time, which can result in new non-work-related social contacts or more participation in physical activities that positively affect health and well-being. "Untangling cause and effect in the relationship between retirement and health is very difficult. Whereas the short-term impacts of retirement on health is somewhat uncertain, the longer-term effects are consistently negative and large."