Aging is a general process of deterioration, and any specific age-related disease, even one of the fatal conditions, is only a very narrow manifestation of that broad deterioration. It is a fantasy to think that any one specific age-related condition can be cured, entirely removed from the full spectrum of damage that is aging, in isolation, and without impact to the rest of aging. The only way to cure an age-related condition is to repair all of the forms of cell and tissue damage that cause it, and each type of damage has widespread effects beyond its contribution to any one named disease. Aging is treated all at once, or not at all, and is treated by addressing the root causes rather than the late disease state, in other words.
This explains how one can arrive at the results of the study noted here. Run the numbers on age-related mortality, remove the contribution of the few top causes of death, and the result is that life is extended by very little. Aged people will shortly die from other causes, given a hypothetical, fantastical way of absolutely preventing mortality attributed to one specific age-related disease in isolation of all of the others.
In the real world, there are ways of affecting, say, cardiovascular mortality to some degree while affecting the progression of other forms of mortality to a lesser degree: statins and antihypertensive medications, for example. But this is isn't the same thing. The reduction of specific forms of downstream damage (atherosclerotic lesions or high blood pressure) causes benefits to mortality that are spread across many age-related conditions, and are thus larger than the numbers in the study here. This is the way that the first rejuvenation therapies will also work, except that they will produce far greater benefits.
To curb the rising global burden of non-communicable diseases (NCDs), the UN Sustainable Development Goals (SDGs) include a target to reduce premature mortality from NCDs by a third by 2030. We estimated age-specific mortality in 183 countries in 2015, for the four major NCDs (cardiovascular diseases, cancers, chronic respiratory diseases, and diabetes) and all NCDs combined, using data from WHO Global Health Estimates. We then estimated the potential gains in average expected years lived between 30 and 70 years of age (LE30-70) by eliminating all or a third of premature mortality from specific causes of death in countries grouped by World Bank income groups. The feasibility of reducing mortality to the targeted level over 15 years was also assessed on the basis of historical mortality trends from 2000 to 2015.
Reducing a third of premature mortality from NCDs over 15 years is feasible in high-income and upper-middle-income countries, but remains challenging in countries with lower income levels. National longevity will improve if this target is met, corresponding to an average gain in LE30-70 of 0.64 years worldwide from reduced premature mortality for the four major NCDs and 0.80 years for all NCDs. According to major NCD type, the largest gains attributable to cardiovascular diseases would be in lower-middle-income countries (a gain of 0.45 years), whereas gains attributable to cancer would be in low-income countries (0.33 years). Eliminating all deaths from the four major NCDs could increase LE30-70 by an average of 1.78 years worldwide, with the greatest increases in low-income and lower-middle-income countries. On average, eliminating deaths from all NCDs (compared with estimates for only the four major types) would lead to a further 25% increase in the gains in LE30-70.