In a fit of exasperation, Miss Manners once demanded of a six-year-old person how it could be so childish and was forced to admit the justice of its reply, “I’m a child.”

Judith Martin (b. 1938) American author, journalist, etiquette expert [a.k.a. Miss Manners]
“Miss Manners,” syndicated column (1978-04-23)
    (Source)

Reprinted in Miss Manners’ Guide to Excruciatingly Correct Behavior, Part 3 “Basic Civilization," "Concerning Children" (1983)