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The correct choice is based on the importance of assessing the timing of psychological impairment in relation to developmental stages. The criterion of "onset before age 18" is crucial because it establishes that the psychological conditions or impairments must manifest during the developmental years. In the context of mental retardation, which is now more commonly referred to as intellectual disability, recognizing that an individual experienced significant challenges during their formative years is vital for diagnosis and understanding the scope of their cognitive and functional limitations.

This criterion ensures that the impairment is not only present but also affects the individual's development at a critical stage in life. If the onset occurred after age 18, it would suggest that the individual was likely functioning at an adequate level prior to that time, which is essential when determining the nature and extent of their mental capacity and support needs. Thus, this emphasis on early onset is integral to the classification and treatment of offenders with intellectual disabilities within the correctional system.