Answer:
"Then Leaf Subsides To Leaf," - symbolizes the child becoming an adult and losing all its innocence and childhood.
"So Eden Sank To Grief," - states that the person realizes that they are no longer a child and that they must face the world all on their own.
"So Dawn Goes Down To Day," - means that the childhood of the person is finished and that it will never return.
"Nothing Gold Can Stay," - states that people change and youth will not stay forever.
Explanation: I believe that is what Robert Frost meant when he wrote these lines. Hope this helps ^-^.