What did Ponyboy mean when he said "Sixteen years on the street and you see a lot. But all the wrong sights, not the sights you want to see" in The...

Johnny is dying too young, and he has had a hard life.

When Ponyboy visits Johnny in the hospital, Johnny is upset because he does not want to die.  He tells Pony that his short life has not been long enough.

"I don't want to die now. It ain't long enough. Sixteen years ain't long enough. I wouldn't mind it so much if there wasn't so much stuff I ain't done yet--- and so many things I ain't seen. …” (Ch. 8)

Ponyboy’s response is that Johnny has seen a lot in his sixteen years, but the sights have been more negative than positive.  Johnny has not had a good life.  In his neighborhood, there is a lot of fighting and not much that is pleasant.  Johnny deserves to live long enough to have an easier life, and see some of the beautiful things he deserves to see.

Johnny is dying because he killed a Soc in self-defense and had to go on the run, and while on the run he hid out in a church that caught fire.  He risked his life and was seriously injuring trying to help a group of children who wandered in.  Johnny was not perfect, but he was a hero, and he did not deserve to die.

Johnny does not want to see his mother.  He has had a hard home life.  His parents are distant or abusive.  For Johnny, who has a sensitive nature, home was not a sanctuary.  His only family are the friends from his gang.

Johnny’s death affects Ponyboy very much.  Johnny tells Pony to “stay gold” because he does not want him to be hardened by the difficult things he sees.  It is too late for Johnny, but Johnny is trying to tell Ponyboy that he can have another life, in a more beautiful world, if he tries.