What would be some of the reasons for a 19 year old failing the montreal cognitive assessment?

The MoCA is used as : a brief test to detect Mild Cognitive Impairment in people 18-99 years old. Often used in older patients with Parkinson's Disease, early Alzheimer's & incipient dementia, it's also used in Vascular Cognitive Impairment (strokes), HIV, Huntington, Multiple Sclerosis, Heart failure, Lung disease, REM sleep disorders, Sleep apnea, Substance abuse, Visual impairment, Brain tumors, Lupus & Epilepsy.