nothing can stay gold forever. What does robert frost mean by this and the loss of innocence

Johnny encourages Ponyboy not to waste his youth looking for things he does't have. Johnny becomes reflective about life using Frost's poem as a metaphor. He tells Pony that life is fleeting and nothing is permanent hence "Nothing Gold can Stay". Johnny encourages Pony to see the goodness in people like Darry instead of wasting their relationship was something it is not. Johnny wants Pony to enjoy the beauty of life while he still ca because, like Johnny's own life, everything comes to an end, especially one's innocence.