This poem that Ponyboy remembers whilst he and Johnny are hiding away in the church is explicitly related to the themes of the novel through its reference to the inevitable changes in nature that correspond to the growth and the loss of innocence of the central characters. This is something that Johnny, in his final letter to Ponyboy, directly comments upon:
I've been thinking about it, and that poem, that guy who wrote it, he meant you're gold when you're a kid, like green. When you're a kid everything's new, dawn... Like the way you dig sunsets, Pony. That's gold. Keep it that way.
The poem thus is an allegory, as viewed by Ponyboy, about growing up and the loss of innocence. Johnny recognises that the gold in the poem relates to a period of relative innocence and of joy in the world that is all to easily lost, as is displayed in the novel in the form of other characters such as Ponyboy's elder brothers and Dally, who have lost the ability to appreciate sunsets because of the harsh nature of their lives. "Nothing Gold Can Stay" is therefore all about the inevitability of growing up and losing that childlike appreciation of the beauty of the world, and this is related to the novel through Ponyboy's struggles not to lose that innocence and not to become cynical and embittered because of the hardships he faces like those around him.
As was mentioned in the previous post, Robert Frost's poem "Nothing Gold Can Stay" allegorically represents growing up and the loss of innocence. Throughout the poem, Frost illustrates the transience of nature and Johnny applies this to Ponyboy and Dally's lives. Johnny understands the importance of remaining innocent and not growing up too fast. He comments on Ponyboy's affinity for sunsets and tells him to stay that way. He also encourages Ponyboy to tell Dally to watch a sunset. Frost's poem illuminates the ephemeral nature of life which is why Johnny understands the importance of staying "gold." He does not want Ponyboy to grow up bitter and lose his childlike innocence. Innocence is "golden" and something to be valued. Johnny also wishes to make Dally aware that there is still hope for his future. Dally is still young enough to change his perspective on life. Similar to how nature is portrayed in Frost's poem, the characters in the novel grow up and struggle to remain "golden."