The author defined aging in terms of the probability of death. I found this definition helpful rather than confusing. You noted that we age and die at a constant rate; how would you define aging if not in terms of the probability of death?<p>There are some problems with the definition. Humans have very high relative probability of death during their first five years, but this quickly declines and then starts to rise again. This means that humans &quot;age&quot; backwards for the first part of their life, which is not particularly intuitive or helpful. However, the definition is helpful in the context of understanding how our aging compares to aging in other animals.