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.
