**Core Concept**
The development of humoral immunity, specifically the production of Immunoglobulin G (IgG), is a critical aspect of the immune system's maturation. IgG is the most abundant antibody isotype in the circulation and provides long-term immunity against infections.

**Why the Correct Answer is Right**
IgG production by the B cells starts at around 6-9 months of age. This is because the development of the immune system is closely linked to the development of the lymphoid organs and the maturation of B cells. The production of IgG is a key indicator of the immune system's ability to mount a specific immune response. The spleen and lymph nodes play a crucial role in the maturation of B cells and the production of IgG antibodies.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because IgG production does not start at birth. Although IgG is the first type of antibody to be produced, it is not present in significant amounts at birth.

**Option B:** This option is incorrect because IgM is the first type of antibody to be produced, not IgG. IgM is produced in response to the first exposure to an antigen and is the primary antibody type present in the newborn.

**Option C:** This option is incorrect because IgA is primarily produced in the mucosal surfaces, such as the gut and respiratory tract, and is not a good indicator of the overall immune system's ability to produce IgG antibodies.

**Option D:** This option is incorrect because IgE is primarily involved in the allergic response and is not a good indicator of the immune system's ability to produce IgG antibodies.

**Clinical Pearl / High-Yield Fact**
The ability to produce IgG antibodies is an important indicator of the immune system's maturity and ability to mount a specific immune response. This is why the ability to produce IgG antibodies is often used as a marker of immune system development in infants.

**Correct Answer:** C.