According to a NewsMax article:
It is not a widely disseminated, downloaded or discussed fact that the average life expectancy for all pro football players, including all positions and backgrounds, is 55 years. Several insurance carriers say it is 51 years.
The article talks about the injuries associated with football, and suggests that these play a role in the early demise of retired players, but I doubt it. The article never comes out and states flatly what the causes of these deaths are so here's my guess: Most of the early deaths are caused by accidents, homicide, or drug and/or alcohol-related problems. Among those who die from natural causes, however, I suspect the leading killer is heart disease brought on by steroid use and excessive weight.
Despite the tacit message of the article which was that retired players are dying from the cumulative effects of injuries they suffered while playing, I doubt that many of them actually do. It would be interesting, though, to see a study done on the cause of death of retired NFL players and to see a comparison of the causes of death by position. My guess is that offensive linemen and defensive tackles, the behemoths of pro football, have the shortest life expectancies among those who die from natural causes.
