The most common external sign of aging is the change in hair color and texture. As people age, their hair may thin and turn gray. Internal changes associated with aging include a decrease in the production of hormones, such as testosterone and estrogen. This can lead to a decrease in muscle mass, bone density, and skin elasticity. Other changes that occur with age include a decreased ability to taste, smell, see, and hear. Additionally, older adults may experience memory problems and changes in mood or sleep patterns.