n. The loss of mental abilities in old age in the opposite order in which they are gained in childhood, especially as exhibited by Alzheimer's patients.
Reisberg and others say that retrogenesis is more than just a newfangled academic term to explain an age-old human condition.

If caregivers understand that Alzheimer's patients are like regressing children and provide the appropriate care depending on the stage, the descent will be less painful, said Reisberg, a professor of psychiatry and clinical director of the Silberstein Aging & Dementia Research Center at New York University.
—John Fauber, “Alzheimer's: Patient, family living life in reverse,” Milwaukee Journal Sentinel, June 23, 2002
1999 (earliest)
Reisberg said researchers from New York University found Alzheimer's patients lose their physical and mental abilities in exactly the opposite order that children gain them and eventually return to an infant-like state. . . . The researchers, who have coined the word "retrogenesis" to describe a patient's collapse, believe the physical suffering of advance Alzheimer's patients could be avoided if they were given the same treatment infants receive.
—“'Second Childishness' is Science,” Reuters, August 18, 1999
The "Reisberg" mentioned in the citations is psychiatrist Barry Reisberg, who invented the term retrogenesis and first presented it to the International Psychogeriatric Association on August 16, 1999.