**Core Concept**
The Human Development Index (HDI) is a composite statistic of life expectancy, education, and per capita income indicators, used to rank countries into four tiers of human development. The age limit for HDI calculation is a critical factor in determining a country's developmental status.

**Why the Correct Answer is Right**
The correct answer is 15 years, as the United Nations Development Programme (UNDP) uses the average of the population aged 15 and above to calculate the HDI. This is because the HDI aims to capture the well-being of the population, and including those aged 15 and above provides a more accurate representation of the population's quality of life. The calculation involves aggregating various indicators, including life expectancy at birth, expected years of schooling, and mean years of schooling, which are all relevant to individuals aged 15 and above.

**Why Each Wrong Option is Incorrect**
**Option A:** 0 years - Including children aged 0 and above would skew the HDI calculation towards the younger population, which may not accurately reflect the overall quality of life.

**Option B:** 30 years - Excluding individuals aged 30 and above would result in an incomplete representation of the population's well-being, as many adults contribute significantly to a country's HDI.

**Option C:** 60 years - Using the age limit of 60 would exclude many individuals who contribute to a country's HDI, particularly those in the workforce and those who have completed their education.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the HDI is a composite index that captures the well-being of a population, and the age limit of 15 years is used to ensure that the calculation accurately reflects the quality of life of the population.

**Correct Answer: C. 60 years**