Determining anyone’s age in Oz can be a difficult matter, as people don’t age unless they want to. (Actually, that was Ruth Plumly Thompson’s idea; L. Frank Baum stated in The Tin Woodman of Oz that even babies don’t age at all, which sounds rather inconvenient.) That means that, for many residents, the age they appear to be is totally different from how many years they’ve lived. For instance, Prince Pompadore of Pumperdink is show celebrating his tenth eighteenth birthday at the beginning of Kabumpo.
So how old, whether in years lived or appearance, are some of the primary characters? It’s tricky not only because of the no-aging thing, but because we aren’t often told when one book takes place in relation to the others. We are told in Tin Woodman, “To judge Ozma of Oz by the standards of our world, you would think her very young—perhaps fourteen or fifteen years of age—yet for years she had ruled the Land of Oz and had never seemed a bit older. Dorothy appeared much younger than Ozma. She had been a little girl when first she came to the Land of Oz, and she was a little girl still, and would never seem to be a day older while she lived in this wonderful fairyland.” I’ve seen some books try to make Ozma even younger, but I fail to see the point of this. We do have some other clues as to the ages of certain characters, including Dorothy. Lost Princess says, “Betsy [Bobbin] was a year older than Dorothy and Trot was a year younger,” and in Giant Horse Trot claims to have stopped aging at ten.
This would presumably make Dorothy eleven and Betsy twelve, and while there’s certainly room to fudge these numbers, I see no reason it would be necessary as far as anything I’ve read goes. Since Dorothy was living in Oz for some time before the other two, she would presumably have been born even before Betsy. The Royal Timeline of Oz estimates 1892 for Dorothy’s birth year. If we go by creator Joe Bongiorno’s dates for Tik-Tok and Scarecrow, I guess Betsy was born in 1900 and Trot in 1903, but this is hardly set in stone.
Dorothy’s age when she first visited Oz is a matter of some debate. In Annotated Wizard, Michael Patrick Hearn guesses that she’s about five or six, based on the idea that each of the first six books except Land takes place a year apart. Of course, there’s no real reason for this assumption, and as Dorothy is returning from a vacation in Dorothy and the Wizard that she was setting out for in Ozma, I would imagine these two, at least, take place less than a year apart. It’s been suggested that Dorothy being able to read in Wizard means she’s been to school, which isn’t to say that plenty of people don’t learn to read before starting school, but would Uncle Henry and Aunt Em have had the time or the inclination to teach her? We know she’s definitely been to school by the time of Ozma.
As for other characters, I indicated in my post on Button-Bright that he “seemed two or three years younger than Dorothy” during his first appearance in Road, but is probably younger than that, and that he’s likely eight or nine when he comes to Oz to live in Scarecrow. His friend Ojo is ten according to Ojo, and the same book claims that he was born after Ozma took the throne in Land.
As reported here, Peter Brown’s age as stated in the books doesn’t really work out mathematically, and it’s been suggested that an editor reduced his age in Pirates. I’m not sure why that would be when some of her other child protagonists are of marriageable age, although these are native Ozites rather than American visitors. Overall, Thompson seemed to be more interested in writing about adolescents than younger children. I don’t know that she ever specifies Speedy’s age, for instance, but he develops crushes on girls during both of his appearances. I discuss Randy’s age in this post, including a mention of how Thompson seems to have bent the rules of Ozian aging a bit so that he could get married in Silver Princess. We don’t really know what the legal age is for marriage in Oz, but considering the time and place in which they were written, I doubt we’re supposed to see, say, twelve-year-olds as potential brides or grooms. The protagonist of Lucky Bucky is twelve; and while I don’t think Jack Snow specifies an age for Twink and Tom of Shaggy Man, Eric Shanower’s “Abby” claims that they were seven at the time. I could go on, but I won’t, at least not right now. I will mention, however, that Cap’n Bill is sixty at the time of Sky Island, making him one of the rare adults for whom an age is specified (even if we don’t know exactly how much time passed between that book and his coming to Oz in Scarecrow).