When Life Goes On, and On ... Re "On Dying After Your Time," by Daniel Callahan (Sunday Review, Dec. 1): Mainstream aging research neither promises radical immortality nor seeks to keep old people sick longer. Aging is a driving factor in the most prevalent and costly chronic diseases. Research indicates that interventions slowing aging delay the onset of these diseases. Therefore, they extend not only life span but also health span, the disease-free and functional period of life. Fundamentally, the goals of aging research are not dissimilar from efforts to prevent or treat Alzheimer's or other chronic diseases in that they both seek to improve quality of life in the elderly. The difference is that interventions in aging may prevent not just one but a range of debilitating diseases simultaneously. The reality is that the world is rapidly getting older. With baby boomers leaving the work force, there won't be enough workers to pay the ever-increasing Medicare costs of the retired. Extending health span will lower Medicare costs and allow aging people to stay engaged. Interventions that slow human aging will provide a powerful modality of preventive medicine: improving quality of life by keeping people healthy and productive while also providing economic benefits for society. BRIAN K. KENNEDY President and Chief Executive Buck Institute for Research on Aging Novato, Calif., Dec. 2, 2013 Daniel Callahan argues against trying to extend the human life span. His arguments are based on three fallacies: life extension will not be accompanied by good health; people of advanced age who continue to work will take jobs away from young people; and Medicare for those over 65 will become exorbitantly expensive. First, there is no evidence that life extension will not be accompanied by good health. Indeed, medical advances increasingly provide better health to the elderly. The coming practice of personalized medicine, stem cell therapy and gene therapy will eventually convert many chronic conditions to curable acute conditions, vastly improving the lives of both the young and the old. Rather than a case of senior citizens taking jobs away from younger people, there will be a larger population whose needs will require a larger work force. As for Medicare, coverage beginning at 65 will be an anachronism. If life extension approaches 100, it is not reasonable for Medicare coverage to begin before 80. Daniel Callahan's essay about "dying after your time," which includes my view, raises important questions about aging, health and longevity. Most have already been answered. Medicare costs will rise as the population ages, but my and my colleagues" research demonstrates that delayed aging would reduce health care costs in the United States by $7.1 trillion by 2060. Postponing death by treating diseases only after they arise will lead to even more dire cost and health projections than those mentioned by Mr. Callahan. If "primary prevention" is the wave of health maintenance in the future, then delayed aging is prevention with a capital P because it would postpone all fatal and disabling diseases simultaneously. Aging research offers the prospect of new ways to live longer more healthfully; life extension is not the goal. Mr. Callahan's concern about older people crowding out younger people for jobs is also unfounded. We saw warnings of this kind before when women began to join the work force. A result was that women added nearly $3 trillion to the economy, and businesses owned by women employ nearly 16 percent of the work force. If health at any age is highly valued, then a healthier older population is worth its weight in gold. Aging science is likely to be the next revolution in public health; it should be embraced, not feared. The writer is a professor of public health at the University of Illinois at Chicago. I applaud Daniel Callahan's essay. I am pushing 86, and I've been a wife, mother, grandmother and great-grandmother. I have taught and studied, earning a Ph.D. I have seen some of Asia and Africa and much of Europe and the United States and have experienced sublime art, music, drama, dance and great literature. I have cooked good food and eaten at three-star Michelin restaurants. What is the way ahead? The medication for my Parkinson's disease lowers my blood pressure, increasing instability and exhausting me. Will another fall break the other hip and switch me from a walker to a wheelchair? I cannot help with my great-grandson as I did with his father; I cannot hold him until I'm seated. The medication also increases the possibility of dementia. My lupus prescription can with time worsen my macular degeneration. I'm too tired and too unstable, and I find it difficult to read a Times recipe to contribute to the family Thanksgiving meal without assistance. What follows to make my full life fuller? Anti-aging research is not, as Daniel Callahan apparently believes, about prolonging the wheelchair-and-walker phase of life but about preserving youthful health and vigor so that there will be far fewer of the elderly in poor health. I'm always amazed at the number of people who vigorously support the search for better prevention and treatment of heart disease, cancer and Alzheimer's but who find moral difficulties in the search for better prevention and treatment of those plus a host of other maladies, all simultaneously. That is what anti-aging research is about. The writer is a professor at the Barshop Institute for Longevity and Aging Studies, University of Texas Health Science Center.