**Core Concept**
The Health and Disability Initiative (HDI) is a framework for assessing and managing health conditions in individuals. The age limit for HDI is a critical factor in determining eligibility for this initiative. The underlying principle being tested is the age-related criteria for initiating HDI.

**Why the Correct Answer is Right**
The correct answer is based on the understanding that HDI is typically initiated in individuals who are 18 years or older. This is because HDI involves comprehensive health assessments, treatment plans, and disability management strategies that are most effective in adults. The 18-year age limit is also a common criterion used in various healthcare settings to determine eligibility for HDI programs.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not specify a clear age limit for HDI. While some healthcare programs may have age restrictions, a vague answer like this is not sufficient to determine eligibility.
**Option B:** This option is incorrect as it suggests an age limit that is too low for HDI. Children and adolescents have different healthcare needs than adults, and HDI programs are typically designed for individuals who are 18 years or older.
**Option C:** This option is incorrect as it suggests an age limit that is too high for HDI. While some individuals may be eligible for HDI at older ages, the typical age limit is 18 years old.

**Clinical Pearl / High-Yield Fact**
It's essential to note that age limits for HDI programs can vary depending on the specific healthcare setting and program requirements. However, in general, 18 years old is the typical age limit for initiating HDI.

**Correct Answer: D. 18 years.**