From Chuck Dinerstein at American Council for Science and Health,
Many of us feel aging is a natural process, after all, everyone “gets it,” and disease is more of a deviation or aberration of nature. The proponents of aging as disease point out that aging is related to the apparent random degradation of our DNA, that aging serves no evolutionary purpose, and is more a “consequence of evolutionary neglect, not evolutionary intent.”  And without an evolutionary role, why consider it natural? Of course, that aging serves no purpose, is a statement made by those with a stake in the outcome, us, and we may be biased in that regard. We may be unaware of aging evolutionary purpose.
From a practical clinical view, what signs and symptoms are associated with aging that differentiates it from other maladies of old age? While science has identified at least nine hallmarks of aging,  each with their signs and symptoms, there are no biomarkers to describe aging overall. Most studies utilize chronological age, and we all know of fit 80-year-olds and decrepit 40-year-olds, so it is too non-specific to be helpful in anything more than a general way. And even if we were to agree on a set of criteria, what is the underlying “cause” that we will want to modify – in truth, it varies for everyone, only when we aggregate do we begin to see some patterns to our declining years.
Why bother classifying aging as a disease?
The short and long answer, money. For scientists, a new disease brings the hope of new funding. In fact, one of the arguments made is that creating the diagnosis of Human Immunodeficiency Virus (HIV) was instrumental in funding research and the subsequent development of effective therapy. More.
How about this approach: Aging neither has an “evolutionary purpose” nor is it a disease. We are transient beings in a transient reality. We live long enough, we wear out, and evolution has little to do with that part of it.
If we do want to talk about evolution, it would be interesting to know why human beings typically live longer than primate apes. Could it be as simple as, we human beings understand the issues around aging as abstractions from reality and can thus take steps to protect ourselves?
See also: Anomaly: Human mortality hits a plateau after 105 years of age Perhaps we can usefully distinguish between a normal human lifespan of 70 to 80 years if all goes well and a maximum human lifespan of around 115-120 years. Because far more human beings than formerly are living out a normal lifespan, more people are available to become centenarians. But transhumanism, the idea that we can use technology to live for centuries, won’t get much support from current longevity data.