## Core Concept
Mental retardation, now more appropriately referred to as intellectual disability, is characterized by significant limitations in both intellectual functioning and adaptive behavior, which covers many everyday social and practical skills. This condition originates before the age of 18. Delayed milestones and slow or retarded growth are indicators that can suggest intellectual disability.

## Why the Correct Answer is Right
The correct answer is based on the definition and diagnostic criteria for intellectual disability. Intellectual disability is typically identified in children who are under the age of 18, with significant delays in reaching developmental milestones and impairments in adaptive functioning. The crucial aspect here is that these delays and impairments must be evident during the developmental period, which is generally considered to be before the age of 18.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incomplete and does not provide a specific age, making it impossible to evaluate its accuracy.
- **Option B:** This option suggests an age that might seem plausible but does not align with the standard definition and diagnostic timeframe for intellectual disability.
- **Option C:** This option is also incomplete and does not offer a specific age for evaluation.
- **Option D:** This option suggests an age of 18 years, which aligns with the definition of intellectual disability, as it specifies that the condition must originate before the age of 18.

## Clinical Pearl / High-Yield Fact
A key point to remember is that intellectual disability is diagnosed based on IQ scores below 70 and significant limitations in adaptive behaviors, with onset before the age of **18 years**. Developmental milestones and growth patterns are critical in early identification.

## Correct Answer: D. 18.