**Core Concept**
Major depressive disorder (MDD) is a chronic mental health condition characterized by recurring episodes of depression. The frequency and duration of these episodes can vary significantly among individuals, and understanding these patterns is crucial for diagnosis and treatment.

**Why the Correct Answer is Right**
Research suggests that the frequency of depressive episodes tends to increase with age in individuals with MDD. This is thought to be due to the cumulative effect of stressors, decreased resilience, and potential changes in brain chemistry and structure that occur with aging. As people age, they may experience more life events that trigger depressive episodes, such as the loss of a loved one, health problems, or social isolation.

**Why Each Wrong Option is Incorrect**
**Option B:** Decrease in length with age - This is incorrect because the duration of depressive episodes can actually increase with age, not decrease. Older adults may experience more severe and longer-lasting depressive episodes.

**Option C:** Last for about 9 months when treated - This is incorrect because the duration of depressive episodes can vary significantly, even with treatment. While some individuals may experience a full recovery within 9 months, others may take longer to recover or experience recurring episodes.

**Option D:** Have a rapid onset - This is incorrect because depressive episodes can have a gradual onset, often developing over weeks or months. While some individuals may experience a rapid onset of symptoms, this is not a characteristic that is common to all depressive episodes.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that MDD is a chronic condition that requires ongoing management and treatment. As individuals age, they may need to adjust their treatment plans to address changes in their mental health needs.

**✓ Correct Answer: A. Increase in frequency with age**