It's more accurate to say that Jasper Johns emerged as a significant and recognized artist in the late 1950s. He was already creating art in the 1940s and early 1950s, but his breakthrough came in the late 1950s with his iconic works like "Flag" (1954-55) and "Target" (1955). This period marked the beginning of his career as a major figure in contemporary art.
It's also important to note that Johns has been continually creating and evolving his art throughout his long career. He is still considered a significant and influential artist today.