Ponyboy will "stay gold" forever because he is selfless and compassionate about others. These qualities do not inhibit one's outer appearance but rather they are manifested inside us. Thus, Ponyboy would remain "gold" because his altruistic and compassionate quality would never fade away.
In the story, there are number of instances when Ponyboy relentlessly showed his selfless character. For instance, he never deserted Johnny nor left him all alone when Johnny was searched by the police.
When Johnny was in the hospital and almost close to death, he said to Ponyboy "Stay gold, Ponyboy, stay gold,”. It shows that Ponyboy was truly a person who was loved by many for his selfless and altruistic nature.
