When Does Aging Become Disease?

In humans, chronological age defines the time since you were born. Whether you are considered old, within your community, depends on many factors. For example, when I grew up in Glasgow a 60-year-old man was very old, while where I sit today, writing this article, is Stockholm – where a 60-year-old man is positively middle-aged. An alternative definition for old-age, articulated by Sanderson and Scherbov (Sanderson & Scherbov, 2013) defines when, for example, you have 10 years of life expectancy left. Thus, the average life expectancy varies from community to community, reflecting a mix of genetics, environment, behaviours and stochasticity.