“Bleak House” (post 5) by Charles Dickens: Harold Skimpole, an adult character who insists he is a child, was said by Dickens to be based on a real person, Leigh Hunt, who apparently had multiple personality

“ ‘You’ll say it’s childish,’ observed Mr Skimpole, looking gaily at us. ‘Well, I daresay it may be; but I am a child, and I never pretend to be anything else’ ” (1, pp. 493-494).

In a letter of 25 September 1853, Dickens stated that Hunt had inspired the character of Harold Skimpole in Bleak House; "I suppose he is the most exact portrait that was ever painted in words! ... It is an absolute reproduction of a real man." A contemporary critic commented, "I recognized Skimpole instantaneously; ... and so did every person whom I talked with about it who had ever had Leigh Hunt's acquaintance” (2).

Comment: When an adult sometimes seriously insists that he is literally a child, he may be speaking from the point of view of his child-aged alternate personality. 

Added same day: So you should ask him, "How old are you?" A child-aged alter will usually have a serious, specific anwer.

1. Charles Dickens. Bleak House [1853]. London, Penguin Books, 2003.

2. Wikipedia. “Leigh Hunt.” https://en.wikipedia.org/wiki/Leigh_Hunt