Aging in Oz is comparable to death in Oz, in that both natural processes have been largely negated by the spell placed on Oz by the fairy queen Lurline.

L. Frank Baum specifies this in the crucial passage on Lurline's spell in The Tin Woodman of Oz, Chapter 12:

From that moment no one in Oz ever died. Those who were old remained old; those who were young and strong did not change as years passed them by; the children remained children always, and played and romped to their hearts' content, while all the babies lived in their cradles and were tenderly cared for and never grew up. So people in Oz stopped counting how old they were in years, for years made no difference in their appearance and could not alter their station.

Yet, as with other such general and blanket statements in Baum's Oz books, this one masks inconsistencies and contradictions. The most blatant contradiction lies in the biography of Princess Ozma, which is crucial to the history and society of Oz. The Wizard of Oz delivered the infant Ozma to the evil witch Mombi, and Mombi enchanted the girl child into a boy. Ozma/Tip, however, did not remain an infant; she/he grew up through childhood, presumably at the normal human growth rate or something close to it. Ozma, of course, is a fairy, so perhaps the rules are different for her than for the humans who existed in Oz when Lurline cast her spell; yet it seems logical that Ozma, as a fairy, should be more resistant to time's passing, not less. I would also like to look at it as the characters grow if they please, but do not age and become "old people". Beginning with Ruth Plumly Thompson, the second Royal Historian, the prevailing view came to be that people in Oz did not age as a matter of course, but could grow older or not as they chose. In her second novel, Kabumpo in Oz, Prince Pompadore celebrates his eighteenth birthday at the start of the book — though it is one of many eighteenth birthdays that he has already celebrated. In The Giant Horse of Oz, both Trot and Prince Philador state that they remain ten years old because they choose to (Chapter 14).

In The Lost King of Oz, Dorothy accidentally wishes herself back to the United States. There, the anti-aging effect of Lurline's spell is not in effect; and Dorothy suddenly begins aging at an accelerating rate — she is turning into the young woman she would then have been if she had remained at home and never gone to Oz. In a panic, she quickly wishes herself back to Oz, where she reverts to her little-girl state (Chapter 10).

The fourteen children in the family of Number Nine have a set schedule: the boys stop growing at the age of twelve, and the girls at ten. The youngest of the brood, Number Fourteen, is still a baby when she is introduced to readers. If procreation continues but people don't die, population growth could become a serious issue; though if the girls stop growing before they reach puberty, that would seem an effective method of birth control. (The Wonder City of Oz, Chapter 5)

It is unclear if outside influence can affect the aging process. The parents of those eternal infants of Oz might someday get tired of changing diapers, and wish that their babies might grow at least through toilet training. Can they do anything to stimulate their babies' growth? Modern Oz author Paul Dana carries one line of thinking to its logical extreme: women who are pregnant when Lurline casts her enchantment over Oz must stay pregnant forever, or leave Oz to give birth. (The Lost Boy of Oz)

The failure of the anti-aging spell is the central premise of Paradox in Oz.

The aging of children under the spell is discussed in the Oz short story "Four Views of General Jinjur."

Eureka the kitten remains a kitten because she gets to Oz and stays there before she grows into a mature cat. (Eureka in Oz)

In Scott Dickerson's The Magic Book of Oz, the anti-aging spell on Oz is effective only while the land is ruled by its rightful rulers, the fairy-inspired line of monarchs named Oz and Ozma. In the days when wicked witches dominated Oz, aging and death returned.

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