The purpose of the definition is not to evaluate individual aging, but the aging of populations. The probability of death neatly summarizes all the risks that t

The purpose of the definition is not to evaluate individual aging, but the aging of populations. The probability of death neatly summarizes all the risks that the population experiences at different ages. Although the probability for a single person is difficult to measure, and can vary wildly with varying behaviour, the probability is easy to measure at the population level. By summarizing over the whole population, you can see the risks that are common to all humans, rather than just particular populations. So smokers or people who jump from planes may not be older compared to those who don't. We don't have a single, reliable instrument that can measure individual level aging.