I do not fundamentally differ with Michael Rose’s definition of aging and his thesis. For him aging is defined as a decline or loss (a “de-tuning”) of adaptation with increasing age, caused by a time-progressive decline of Hamilton’s forces of natural selection. To my mind, this definition is consistent with most previous definitions of aging used by the majority of evolutionary biologists.

Most evolutionary biologists define aging as an age-dependent or age-progressive decline in intrinsic physiological function, leading to an increase in age-specific mortality rate (ie, a decrease in survival rate) and a decrease in age-specific reproductive rate (eg, Medawar, 1955; Williams, 1957; Rose, 1991; Partridge and Barton, 1996; Tatar, 2001; Promislow and Bronikowski, 2006; Flatt and Schmidt, 2009; Bronikowski and Flatt, 2010; Fabian and Flatt, 2011). Rose (1991), for example, in his seminal book on the evolution of aging defines aging as “a persistent decline in the age-specific fitness components of an organism due to internal physiological degeneration.” At level of the individual, the intrinsic physiological state at a specific age determines, among other things, whether an individual is dead or alive and how much it reproduces. At the level of the cohort, the underlying physiological states of the individuals translate into the age-specific rates of mortality and reproduction. We therefore diagnose demographic aging to occur if we observe an increase in age-specific mortality and a decrease in age-specific reproductive rate in the cohort (eg, Bronikowski and Flatt, 2010).