As people progress through middle adulthood, they experience (primary aging) in the form of molecular and cellular changes in the body. wrinkles and gray hair are visible signs and less visible signs include arthritis and a gradual dulling of the senses of taste, smell, touch, and vision.

I actually think that as people progress through middle adulthood, they experience Senescence (primary aging) in the form of molecular and cellular changes in the body. In this process cells irreversibly stop dividing and enter a state of permanent growth arrest without cell death. The process plays roles in normal development, maintains tissue homeostasis, and limits tumor progression.