An increasing interest in intervening in the aging process, in treating aging as a medical condition, inevitably produces a greater interest in measuring aging. When is an individual old? Even in the absence of new biotechnologies of rejuvenation, people clearly age at different rates, the result of lifestyle choices such as exercise and weight, exposure to persistent pathogens, and related factors. One sixty year old can be more aged or less aged than another. It might seem a little academic to be debating today how to determine whether or not someone is old, but given the ability to actually produce rejuvenation - such as via senolytic drugs that clear senescent cells from old tissues - this becomes a much more practical concern.
One widely used measure of population aging is the potential support ratio, the inverse of the old age dependency ratio. The potential support ratio divides the population 20 years of age and older into two disjoint age groups. Conceptually, the ratio is meant to reflect the stages of the human life cycle, distinguishing between adults who are elderly and those who are not. To compute the ratio two sorts of information are needed, the number of people at each age starting from 20 and a threshold age that divides the adult population into a group who are elderly and a group who are not.
On its website Profiles of Ageing 2019 the UN now publishes a conventional potential support ratio (PSR) and a prospective potential support ratio (PPSR). The difference between the two variants is based solely on different threshold ages at which people first become categorized as"old" . In the PSR that threshold age is age 65 and is fixed independent of time or place. In the PPSR the threshold age is the age where remaining life expectancy is 15 years. We call the first, the conventional old age threshold (COAT) and the second the prospective old age threshold (POAT). The COAT is the most commonly used old age threshold, but it has the disadvantage that it does not change over time and is the same for all countries regardless of their trajectories of aging.
The choice of whether to use the COAT or the POAT in assessing the extent of population aging is not arbitrary. It is not like choosing between Celsius and Fahrenheit in the measurement of temperature. Having measures of population aging based on the COAT and the POAT is like having two kinds of thermometers, where sometimes both indicate that the temperature is increasing and sometimes one indicates it is getting warmer while the other indicates it is getting cooler. Indeed, sometimes measures of population aging based on the two old age thresholds change in the same direction and sometimes they do not.
We propose that the old age threshold should be determined using an equivalency criterion - in other words, people at the old age threshold should be roughly similar to one another in terms of relevant characteristics regardless of when and where they lived. Using historical data on five-year death rates at the old age threshold as an indicator of one aspect of health, we assessed the extent to which the two approaches used by the UN are consistent with the equivalency criterion. The results indicate that the old age threshold based on a fixed remaining life expectancy is consistent with the equivalency criterion, while the old age threshold based on a fixed chronological age is not.