What we consider to be the oldest novel in world literature really depends on the strictness of our definition of a novel. The looser the definition, the farther back we can go.
For example, the Japanese work the Tale of Genji(11th century A.D) by Murasaki Shikibu is often cited as the oldest novel in the world. But if you were to read the Tale of Genji, you might find it very different in nature to modern novels you are used to reading.
The modern novel is a specific type of prose fiction that only really became popular and recognized as a distinct form in the 18th century. We have now come to expect that a novel will include certain elements, including a plot, a protagonist or narrator, some central characters, possibly some central themes, and a conflict that may or may not be resolved. Many novels contain a climax near the end.
The Tale of Genji is wonderful literature, especially in Arthur Waley's idiomatic translation, but the plot is meandering, it does not have an ending, and the narrative is frequently interrupted by hundreds of poems. In order to include the Tale of Genji as a candidate for world's oldest novel, we are taking a relatively broad view of what we consider to be a novel. Under this broad definition, the Satyriconwritten by Petronius in the 1st century A.D. would easily beat the Tale of Genji to the title of world's oldest novel.
Under a stricter definition, Don Quixote (1605–1615) by Miguel de Cervantes might be considered the world's first novel.
A book that many consider to be the world's first "modern novel" is Robinson Crusoe by Daniel Defoe, which was first published in 1719. Robinson Crusoe marks the beginning of realistic fiction in world literature, and is certainly another contender for the world's first novel depending on the strictness of our definition.