Let us count the ways in which biochemical processes, optimized through evolution for youthful health and reproductive success, interact poorly as they change with age, and thus cause pain and suffering. Here is one example of many from EurekAlert!: "Jason Dyck and his research team at the University of Alberta have been studying the types of fuels used by the heart in young and aged mice. The young healthy heart normally used a balance of fat and sugar to generate energy to allow the heart to beat and pump blood efficiently. However, as the heart ages the ability to use fat as an energy source deteriorates. This compromises heart function in the elderly. Interestingly, at a time when the heart is using less fat for energy, Dyck has shown that a protein that is responsible for transporting fat into the contractile cells of the heart actually increases. Based on this finding, Dyck proposed that the mismatch between fat uptake and fat use in the heart could lead to an accumulation of fat in the heart resulting in an age-related decrease in heart function." This being the age of biotechnology, they go on to engineer a mouse that doesn't suffer from this specific problem - but you get the idea. Aging is damage and change that causes disarry; repair or prevent the changes sufficiently well and you don't have to patch up after the disarray.