Aubrey and Paul,
Both of you have supported what I thought -- that at the time Terminal Dementia was a catch-all categorization for a spectrum of psychiatric symptoms.
The man in question could have had brain damage at birth and just become too difficult to manage at home as an adolescent. He could have developed symptoms later. There's no way of knowing as none his family who would have known is living. In addition, when Essex County Hospital for the Insane moved to Cedar Grove, NJ and became Overbrook in the early 20th century, his records probably went with him. Overbrook is now razed, and pictures from its abandonment show that patient records were left in the basement to rot. There may be court records of his original committal as the family had to post a "bond" for his care at the institution.
Thanks for replying.
Smyrna, DE USA