**Core Concept**
Depression is a mood disorder characterized by persistent feelings of sadness, hopelessness, and loss of interest in activities. The age of onset is a critical factor in understanding the clinical presentation and management of depression.

**Why the Correct Answer is Right**
The most common age for depression is during middle age, typically between 45 and 65 years. This is often referred to as late-life depression. Several factors contribute to the increased risk of depression in this age group, including the loss of a spouse, retirement, and chronic medical illnesses such as heart disease and diabetes. The hypothalamic-pituitary-adrenal (HPA) axis plays a crucial role in the pathophysiology of depression, and aging is associated with changes in the HPA axis, which may contribute to the increased risk of depression.

**Why Each Wrong Option is Incorrect**
**Option A:** Early adulthood is a common age for depression, but it is not the most common age. This age group is often associated with depressive episodes triggered by significant life events such as graduation, job change, or relationship problems.

**Option B:** Childhood depression is a significant concern, but it is not the most common age for depression. Children and adolescents with depression often present with irritability, anxiety, and behavioral problems.

**Option C:** Old age is associated with a higher risk of depression, but it is not the most common age. The risk of depression increases with age, but the majority of individuals with depression are middle-aged.

**Clinical Pearl / High-Yield Fact**
Depression is a common comorbidity in patients with chronic medical illnesses, and it can significantly impact treatment outcomes. Clinicians should be aware of the increased risk of depression in patients with conditions such as heart disease, diabetes, and stroke, and consider screening for depression in these patients.

**Correct Answer:** C. 45-65 years.