Eighty-year-old Lucia has difficulty remembering to call her son every Sunday at noon. This is most likely due to a decline in her:

The brain is an important factor concerning memory. There are certain proteins and hormones that protect brain cells and fix damaged ones, as well as spark neural growth. However, as we age, these proteins and hormones decline, resulting in less cell reparation and neural growth (which correlates to memory loss). So, Lucia is probably experiencing this decline.