The story happens in the early twentieth century although Jody's grandfather makes reference to his own youth (the 1800s) and experiences out West. He says that he had kept travelling west and would have gone further but for the fact that he was stopped by the Pacific Ocean. (Actually, he could have taken a boat and gone on to Asia if he had wanted to, but he said this for rhetorical effect.)
Here is some background information concerning Salinas Valley where Jody's family lived:
Salinas was not always an agricultural area. Initially, it was the home to several small tribes of Native Americans, who lived there for many thousands of years before Spanish soldiers and missionaries arrived. Under Spanish rule, the main focus of the population was the coastal areas, and so the valley, where Salinas is located, was largely left on its own. However, when the Mexicans overthrew the Spanish rulers, the Mexican government began giving out land grants. From these land grants grew the communities that would eventually make up such towns as Salinas.
During the early stages of the gold rush, James Bryant Hill bought a large land grant in the valley and was one of the first people to plant crops there. It would not be until 1867 that a partnership between a few large farmers and cattle rancher Eugene Sherwood would be formed. These men laid out a plan for a half-mile square that would become the heart of Salinas City.
At the time of the writing of The Red Pony, wheat was still the main crop grown in the valley. Most of the land was still used for cattle. Today, Salinas is known for its crops of lettuce, artichokes, broccoli, sugar beets, and beans. Due to its rich valley soils, Salinas would eventually become one of the wealthiest cities per capita in the United States.
- from eNotes.com/red-pony/historical-context
Salinas Valley was Steinbeck's own stomping ground for most of his life, and his love for California comes through many of his works.