What Is the Onset Age of Human Aging and Old Age?

Humans are complex beings in whom diverse biological, psychological, and social elements, which are interdependent, combine together and complement each other. Therefore, human aging is different from cellular aging and organ or system aging. In addition, as noted by Leonard Hayflick1 “it is not the mere passage of time; but the manifestation of biological events that occur during a period of time, which defines aging”; hence, “aging occurs over time, but not because of the passage of time.” In this sense, most of the cells that are present in our body now did not exist 5 or 10 years ago, or even 2 days ago; this phenomenon occurs in all stages of life (e.g., in children, adults, and the elderly). Therefore, the cells that were renewed and replaced by new ones in the past decade are younger in all age groups, including the elderly. However, the paradoxical question that arises is the following: if most of the cells of aged people are renewed over time, and therefore remain younger, can the elderly rejuvenate over time? Of course, the answer is no. In this context, we define human aging as a gradual and adaptive process, characterized by a relative decrease in the biological reserve and response to demands to maintain or restore homeostasis, due to morphological, physiological, biochemical, psychological, and social changes, prompted by genetic load and accumulated wear, and the challenges that a person faces throughout their life in a given environment2. There is no consensus in the gerontological field regarding the age at which aging begins; however, some authors, based on a time-limited approach, have established that aging begins at birth or even from conception. We assume that aging starts from the 5th decade of life, at about 45 years of age, as biological, physical, psychological, and social changes, which are evident and related to the aging process, occur in the majority of the population at this stage of life. This does not mean that we shall ignore that human aging is a complex, multidimensional, and multifactorial process, and therefore individualized; however, a starting point of aging should be assumed for the implementation of social and community programs aimed at this section of the population. By contrast, chronological age is usually considered as a parameter to indicate at what point of time a person becomes old. In this