This is an observation made by Ponyboy after he sees Sodapop sleeping and thinks he looks younger than 17 years old. He had also noticed that Johnny looked younger when sleeping as well.
In considering the meaning of the statement in the context of the book, it is important to note that the important people in Ponyboy's life, "the Greasers," have all had to grow up quickly and deal with adult responsibilities. This can age people. In their sleep, however, they are not dealing with the struggles of survival and keeping family together; therefore, they may look younger and more peaceful.
This is the kind of question that only you can answer because only you know what you think. I'll give you my opinion, and maybe that will help you think about it. Have you ever heard the song "These Dreams" by Heart? There is a line that says in dreams we "live a thousand lives." In my dreams, my parents are still married, and my father is still alive, and we all still live together in the family home. I'm still somebody's child instead of a middle-aged woman. I can do the things I wanted to do and live the life I thought I would live when I was younger.
Or maybe you can think about how people look when they are asleep. Think of the expression "sleep like a baby." The cares of the day disappear and there is a peaceful look on a sleeping person's face.
In my opinion, people look younger when they are asleep becuase when you sleep, all your worries, your thoughts, the stress would be gone and you would rest peacefully. However, that's only in middle ages, once you're an adult, there are just too many things to worry about that you can't really have a good sleep (especally, the moms with children gets more stressed which therefore, they won't look young)
at the end of the book it says " pony boy likes dally, they become something fuckeachother cetch hiv form anal ya bye :)