We teach our children to walk, talk, feed themselves, bathe, and act properly in public.
Teens learn manners and self control.
All this learning is stored on nerve cells. Those human skills are lost forever as dementia kills brain cells. The dying brain, then, causes adult behavior to reverse. Patients in the middle stage of dementia function at the level of a 5-10 year old child. End stage patients function like an infant or toddler.