**Core Concept**
Hodgkin's disease is a type of lymphoma characterized by the presence of Reed-Sternberg cells, and its prognosis is influenced by various factors, including age, systemic symptoms, histological subtype, and disease extent.

**Why the Correct Answer is Right**
Younger age is actually a **good** prognostic factor for Hodgkin's disease. Patients under the age of 40 tend to have a better response to treatment and higher survival rates compared to older patients. This is because younger patients are more likely to have favorable histological subtypes and fewer comorbidities.

**Why Each Wrong Option is Incorrect**
**Option B:** Systemic manifestations, such as fever, weight loss, and night sweats, are poor prognostic factors for Hodgkin's disease. These symptoms indicate a more advanced disease and a higher risk of relapse.
**Option C:** Lymphocyte depletion is a subtype of Hodgkin's disease characterized by a lack of lymphocytes and the presence of Reed-Sternberg cells. It is associated with a poor prognosis and a higher risk of relapse.
**Option D:** Mediastinal disease refers to Hodgkin's disease that involves the mediastinum, a region in the chest. Mediastinal disease is often associated with a poor prognosis, as it can be more difficult to treat and may be more likely to cause complications.

**Clinical Pearl / High-Yield Fact**
When assessing a patient with Hodgkin's disease, it's essential to consider their age, systemic symptoms, and histological subtype to determine their prognosis. Younger patients without systemic symptoms and with favorable histological subtypes tend to have a better prognosis.

**✓ Correct Answer: A. Younger age**