Stan Lee joined Timely Comics in 1939 at the age of 17. He was eventually promoted to editor, and later became publisher. He played a major role in developing the characters and stories that became synonymous with Marvel Comics, including Spider-Man, the Fantastic Four, the X-Men, and many more.
However, it's important to recognize that Marvel Comics existed before Stan Lee joined the company, and he wasn't the sole creator of the Marvel Universe. He worked with many talented artists and writers to build the foundation of the company that we know today.