## Core Concept
The frontal sinuses are part of the paranasal sinuses and are known for their variable development among individuals. Their growth and pneumatization are influenced by genetic and environmental factors. The development of frontal sinuses is often used as an indicator of sinus maturity in medical and forensic evaluations.

## Why the Correct Answer is Right
The frontal sinuses typically begin to develop around the second year of life and continue to grow and pneumatize throughout childhood and adolescence. The development may not be complete until later in adolescence. Studies and clinical observations suggest that the frontal sinuses may not be fully developed until around 6 to 8 years of age, but significant variability exists. The maximum age up to which they may not be developed normally is often cited as around 6 years, reflecting the range of normal development.

## Why Each Wrong Option is Incorrect
- **Option A:** 2 years. This is too early for the completion of frontal sinus development, as they start developing around this age but are not fully formed.
- **Option B:** 4 years. Still relatively early for the full development of frontal sinuses, as they continue to grow throughout childhood.
- **Option D:** 12 years. While frontal sinuses continue to grow and pneumatize during adolescence, most are sufficiently developed by the age given in the correct answer, making this option less accurate for the maximum age up to which they may not be developed.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the development of frontal sinuses can be an important factor in the diagnosis and management of sinusitis in children. Their incomplete development can affect the presentation and treatment of sinus infections. Clinicians often consider the age of sinus development when evaluating and treating sinus-related conditions in pediatric patients.

## Correct Answer: C. 6