How is Ponyboy choosing between growing up too soon or never growing up a coming-of-age event in "The Outsiders"?
Ponyboy begins this story as an idealistic and dreamy kid. He likes sunsets and poetry and is often teased by his friends for being too "brainy". The other teenage boys in his group are more hardened - Dally has been to jail, Sodapop has dropped out of school to work, Darry put off college to work, Johnny is abused at home and beaten badly by the Socs. Ponyboy, though, is somewhat sheltered. He hasn't had an easy life, but he has been somewhat protected.
After Johnny and Dally die, Ponyboy becomes more hard edged. His grades at school drop and he instigates a fight, even breaking a bottle to use as a weapon. Two-Bit tries to tell him that he is not acting like himself, but Pony doesn't understand. Then he finds Johnny's last letter to him. Johnny talks about "staying gold", and encourages Pony to do just that. He wants Pony to hold on to the good in life and not let the bad stuff corrupt him - not too grow up to fast.
Coming-of-age means a maturing process. It doesn't necessarily mean "growing up" and losing innocence, it means being mature enough to see the difference. By recognizing that it is ok to be young and protected, Ponyboy is showing that he is mature enough to choose for himself - an adult move.