He means for Ponyboy to hold on to his innocence and youth as long as he can. The source of quote is a poem by Robert Frost, "Nothing Gold Can Stay."

In the novel The Outsiders by S. E. Hinton, Johnny tells Ponyboy to "stay gold," which means to stay out of trouble and follow his dream. The "stay gold" is a reference to the poem Johnny read to Ponyboy earlier in the novel which is a poem by Robert Frost called "Nothing Gold Can Stay". Johnny knows that Ponyboy doesn't really belong in a gang, and that he could follow his dreams as the others cannot. Johnny does not want Ponyboy to end up like him, a hero who saved children but dies from the injuries he sustained while doing so. He wants Ponyboy to be the golden boy of the group and succeed at following his dreams.