**Core Concept**
The question requires knowledge of the development and maturation of lymphoid tissues, including lymph nodes, spleen, and lymphoid organs. Lymphoid tissues play a crucial role in the immune system, particularly in the defense against pathogens and the maintenance of immune homeostasis.

**Why the Correct Answer is Right**
Lymphoid tissues reach their maximum size during puberty due to the rapid growth and development of the immune system. During this period, there is an increase in the number of lymphocytes, including T cells and B cells, which are essential for the adaptive immune response. The enlargement of lymphoid tissues is also attributed to the increased production of antibodies and the activation of immune cells. This process is mediated by the thymus, which is a key organ involved in the maturation of T cells.

**Why Each Wrong Option is Incorrect**
**Option A:** While lymphoid tissues do develop early in childhood, they do not reach their maximum size during this period. In fact, the thymus, which is a crucial organ for T cell maturation, is most active during childhood and begins to involute (shrink) after puberty.

**Option B:** Adolescence is a period of significant growth and development, but it is not the time when lymphoid tissues reach their maximum size. While some growth and maturation of the immune system occur during adolescence, the peak growth and development of lymphoid tissues occur earlier.

**Option D:** Lymphoid tissues do not reach their maximum size at 20 years of age. While the immune system continues to mature and develop throughout young adulthood, the peak growth and development of lymphoid tissues occur earlier, during puberty.

**Clinical Pearl / High-Yield Fact**
The thymus, a key organ involved in T cell maturation, begins to involute (shrink) after puberty, which is why it is often smaller in adults. This process is a normal part of immune system development and is not indicative of any underlying disease or disorder.

**✓ Correct Answer: C. At pubey**