Although no one says these lines out loud, Ponyboy says them to himself, in his mind. It's something he's thinking while talking with Johnny, right after Johnny had said that sixteen years isn't long enough to live, and that Johnny wishes he could see and do more things, especially travel beyond their neighborhood more, before dying. Ponyboy silently agrees, and even though he's also thinking that sixteen years as a gang member means that you see things you shouldn't see, he doesn't voice these thoughts out loud: he doesn't share them with Johnny. It's important for Ponyboy not to upset Johnny, and not to get too emotional.
You can find this discussion a little less than halfway through Chapter 8, as Johnny is lying in critical condition in the hospital after saving the kids from the fire. The exact page number will be different depending on which version of the text you have, but if you open to Chapter 8 and scoot past the conversation that includes Two-Bit, then you've found the conversation that Johnny and Ponyboy have alone. And if you scan for the beginning of a paragraph that starts with the words "Sixteen years," then you've found it. If you've reached some longer paragraphs that mention Two-Bit again, plus Johnny's mother, then you've gone too far--scoot back to where the paragraphs are short and contain a lot of quotation marks to indicate the conversation.