## Core Concept
The Juvenile Justice Act is a law that aims to consolidate and amend the law relating to juveniles in conflict with law and children in need of care and protection. The definition of a juvenile under this act is crucial for determining the jurisdiction and applicability of the law. The act specifies the age limit for considering a person as a juvenile.

## Why the Correct Answer is Right
The Juvenile Justice (Care and Protection of Children) Act, 2015, defines a juvenile or a child who has not completed eighteen years of age. This definition is critical in the context of criminal law and child protection services, as it differentiates the legal procedures and protective measures for individuals under eighteen from those who are adults.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not accurately reflect the age definition provided by the Juvenile Justice Act.
- **Option B:** This option is incorrect for the same reason as option A; it does not align with the legal definition of a juvenile.
- **Option C:** Although not directly provided, if this option does not state eighteen years of age, it would be incorrect based on the accurate definition provided by the act.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the Juvenile Justice Act of 2015 raised the age of a juvenile from 16 to 18 years for boys. This change ensures that all children, irrespective of gender, are treated equally under the law, with the age of 18 being the threshold for considering someone a juvenile.

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