What is the term for being declared unable to understand due to mental illness or impairment?

The term "legally incompetent" refers to an individual's inability to understand or manage their affairs due to mental illness or impairment. In legal contexts, a person declared legally incompetent lacks the mental capacity to make informed decisions or comprehend the nature and consequences of their actions. This determination often leads to the appointment of a guardian or conservator to manage their legal and financial matters, ensuring that the individual's rights and welfare are protected.

The other options are not appropriate for describing the state of being unable to understand due to mental illness. A mental institution refers to a facility where individuals with mental health issues may receive treatment, not the status of understanding. Association typically implies a group or relationship between entities and does not relate to legal competency. An advocate represents or supports someone, but does not describe the condition of understanding mental capacity. Thus, "legally incompetent" is the most accurate term for the context provided.