When did James Gatz change his name to Jay Gatsby?

James Gatz changed his name when he was seventeen-years-old.

James Gatz — that was really, or at least legally, his name. He had changed it at the age of seventeen and at the specific moment that witnessed the beginning of his career — when he saw Dan Cody’s yacht drop anchor over the most insidious flat on Lake Superior. It was James Gatz who had been loafing along the beach that afternoon in a torn green jersey and a pair of canvas pants, but it was already Jay Gatsby who borrowed a rowboat, pulled out to the Tuolomee, and informed Cody that a wind might catch him and break him up in half an hour.