**Core Concept**
The question is testing the concept of age estimation in forensic dentistry, specifically the use of amino acids in root dentin to determine the age of an individual.

Amino acids in root dentin are used as markers for age estimation because their concentration and composition change over time due to the process of dentinogenesis. This process involves the gradual replacement of dentin matrix proteins with minerals, leading to changes in the amino acid profile.

**Why the Correct Answer is Right**
The correct answer is Hydroxyproline, which is a non-essential amino acid that is high in root dentin and used for age estimation. Hydroxyproline is a key component of collagen, a major protein in dentin matrix. As collagen is broken down and replaced over time, the concentration of hydroxyproline in root dentin increases, making it a useful marker for age estimation.

The mechanism behind this is the gradual degradation of collagen, which is catalyzed by enzymes such as collagenase. As collagen is broken down, the resulting peptides are then mineralized, leading to an increase in hydroxyproline concentration.

**Why Each Wrong Option is Incorrect**
**Option A:** Glutamic acid is not a primary amino acid used for age estimation in root dentin.
**Option B:** Histidine is not a significant marker for age estimation in root dentin.
**Option C:** Leucine is not a primary amino acid used for age estimation in root dentin.

**Clinical Pearl / High-Yield Fact**
Hydroxyproline is a useful marker for age estimation in forensic dentistry because its concentration in root dentin increases with age, making it a reliable indicator of age.

**Correct Answer: C. Leucine is not correct, however, the question asks for the amino acid that is high in root dentin used for age estimation is and the correct answer is Hydroxyproline but it was not listed, the correct options were not provided. However, I will provide the correct answer as per the options provided. As the correct answer was not provided, I will provide the explanation for Hydroxyproline.**

In case the correct options were provided, the correct answer would be: **Correct Answer: A**