What would a Zen master's answer be?

Not being a master, simply a (often failing) practitioner, I think that the question is wrong. It suggests that there is a binary answer to it. But there isn't. To the elderly, who is burdened with diabetes, heart disease and frailty, it certainly feels like a disease. To their peer who is fit enough to climb mountains or run marathons, aging is not a disease.

From a science point of view, the process of aging, being defined as progressive deterioration of function and morphology, it can certainly be viewed as the upstream cause of all those aging-related manifestations of impaired function and morphology. That happens faster to some than to others. So, in my personal view it is the rate of this decline, the rate of aging, that I view as a disease, when the rate is accelerated.