Scientific evidence links physical activity to several benefits. Recently, we proposed the idea that exercise can be regarded as a drug. As with many drugs, dosage is of great importance. However, to issue a public recommendation of physical activity in aging is not an easy task. Exercise in the elderly needs to be carefully tailored and individualized with the specific objectives of the person or group in mind. The beneficial effects of exercise in two of the main age-related diseases, sarcopenia and Alzheimer's Disease, are dealt with at the beginning of this report. Subsequently, dosage of exercise and the molecular signaling pathways involved in its adaptations are discussed. Exercise and aging are associated with oxidative stress so the paradox arises, and is discussed, as to whether exercise would be advisable for the aged population from an oxidative stress point of view. Two of the main redox-sensitive signaling pathways altered in old skeletal muscle during exercise, NF-κB and PGC-1α, are also reviewed. The last section of the manuscript is devoted to the age-associated diseases in which exercise is contraindicated. Finally, we address the option of applying exercise mimetics as an alternative for disabled old people. The overall denouement is that exercise is so beneficial that it should be deemed a drug both for young and old populations. If old adults adopted a more active lifestyle, there would be a significant delay in frailty and dependency with clear benefits to individual well-being and to the public's health.