What does Johnny mean by this piece of advice?

Before he dies, Johnny whispers to Ponyboy, "Stay gold, Ponyboy. Stay gold..." He is referencing the poem by Robert Frost that Ponyboy recited aloud when they were sitting on the back porch of the church, watching the sunrise. By dying, Johnny is fulfilling the prophecy of the poem that "Nothing gold can stay." But he hopes for Ponyboy to fulfill his own potential and not wind up like him.