Final answer:
The factor that would decrease a person's basal metabolism is Aging. As people age, particularly after 30, and more drastically between 50 and 70 years old, muscle mass decreases, lowering the metabolic rate and leading to less physical activity, which further reduces metabolism.
Step-by-step explanation:
Aging can lead to a decrease in a person's basal metabolism. The human body's metabolic rate decreases nearly 2 percent per decade after age 30, primarily due to changes in body composition such as reduced lean muscle mass. Between the ages of 50 and 70, there is a more significant loss of muscle mass, which causes a more pronounced decline in metabolic rate. Reduced muscle mass leads to less strength and can result in lower levels of physical activity. This, in turn, creates a cycle where decreased activity leads to further muscle loss, reducing the metabolic rate even more.