I spent decades working in nursing homes. Every one of them had lots of residents with cognitive impairment. Very, very few of them were born with it. The dementia we see in many of the very elderly is a form of cognitive impairment. Most cognitive impairment is due to strokes, atherosclerosis, Alzheimer's, Parkinson's and other diseases associated with aging. Physical trauma to the head is a cause, as we see with athletes in football and boxing. It certainly can present in infancy, due to problems in utero and ischemia during the birthing ordeal, as well as due to genetic disease.

Dementia is a very serious level of cognitive impairment. Less severe levels of cognitive impairment might not be incompatible with being able to write some coherent prose.

One of my patients was a lovely elderly lady who seemed to be just fine cognitively. She had worked in a girls' boarding school, and she still exchanged letters with students who had attended that school. I couldn't understand why her diagnoses included dementia. One day she mentioned that her hip was a bit sore. I said, "Well, that's probably related to you breaking your hip and getting that hip surgery." She responded, "Did I break my hip?" She had no recollection of falling and needing hip surgery. I'd known her for two years and was utterly shocked that she could not remember breaking her hip. So there are many permutations of cognitive decline, and some are subtle, and some are not readily apparent.