**Core Concept:** Osteosarcoma is a malignant bone tumor originating from mesenchymal cells, most commonly affecting long bones in children and adolescents.

**Why the Correct Answer is Right:** Osteosarcoma primarily occurs in children and adolescents due to an increased rate of cell division and growth during this phase of life. Additionally, the rapid growth and remodeling of bones during this age group increase the chances of genetic mutations leading to tumor formation.

**Why Each Wrong Option is Incorrect:**
A. Young adults (18-30 years): While osteosarcoma can occur in young adults, it is less common in this age group compared to children and adolescents.
B. Elderly adults: Osteosarcoma is less common in older adults, as they have a lower rate of cell division and growth.
C. Infants and very young children: Osteosarcoma is less common in this age group due to slower bone growth and lower cell division rates.
D. Adolescents and young adults (10-20 years): Although osteosarcoma can affect this age group, it is less frequent compared to children and adolescents due to the higher growth rate and cell division during this phase of life.

**Clinical Pearl:** Osteosarcoma is a rare but aggressive tumor that requires prompt diagnosis and treatment. The early detection and management of osteosarcoma in children and adolescents can lead to better outcomes and increased survival rates.

**Correct Answer:** **C. Infants and very young children**

Osteosarcoma is less common in these age groups due to slower bone growth and lower cell division rates, making it less likely for genetic mutations and subsequent tumor formation.