This is not directly alluded to or stated in either the film or the various Oz books. I hasten to add I am not, and never was an Oz fan. These fantasies are so far removed from everyday life that attempts at ( realism) like such things as addresses and ages simply do not apply, or were not applied! She resides on a farm in Kansas, living with her aunt and uncle. I would imagine the story begins in the summer time, as there is not mention of school and she would appear to be of school age- certainly not a preschool tot. People do frequently ( more common in the old days- let us not forget that Oz was written in l900- ( Farm out) -no pun intended , kids with relatives in the off season, this may apply. But her age is never directly stated. It has been argued some of the Monster characters are distorted versions of people in her normal social orbit- such as farmhands, etc. but this is not proven.
No birthdate is given for Dorothy Gale in "The Wizard of Oz."
Specifically, author and Oz series creator Lyman Frank Baum (May 15, 1865 - May 6, 1919) gives no information as to when and where Dorothy Gale was born. Neither does he give any information as to her age. From the illustrations in the original 1900 book edition, readers would assume Dorothy to be a young girl of elementary school age.
Dorothy Hamill's birth name is Dorothy Stuart Hamill.
Dorothy Odam was born on March 14, 1920, in England, UK.
Dorothy Lavinia Brown was born in 1919.
Dorothy Rudd Moore was born in 1940.
Dorothy LeMay was born on February 14, 1954.
Dorothy Fay's birth name is Dorothy Alice Fay Southworth.
Fay Holden's birth name is Hammerton, Dorothy Fay.
Dorothy Fay died on November 5, 2003, in Woodland Hills, Los Angeles, California, USA of natural causes.
Eliza Fay was born in 1756.
John Fay was born in 1773.
Maude Fay was born in 1878.
Johnny Fay was born in 1966.
Fay Zwicky was born in 1933.
Fay Gale was born in 1932.
Ming Fay was born in 1943.
Fay Chung was born in 1941.
Fay Davis was born in 1873.