The boys age, only peter pan doesn't. Peter pan also HATES adults. Infact... hate may not be a strong enough word.
From the book itself:
“The boys on the island vary, of course, in numbers, according as they get killed and so on; and when they seem to be growing up, which is against the rules, Peter thins them out; but at this time there were six of them, counting the twins as two.”
This explanation occurs in the beginning part of chapter 5.
http://www.gutenberg.org/files/16/16-h/16-h.htm
It is also explained that peter has no guilt because peter has no memory. Or more specifically very very limited ability for memory.
As per chapter 17
She had looked forward to thrilling talks with him about old times, but new adventures had crowded the old ones from his mind.
“Who is Captain Hook?” he asked with interest when she spoke of the arch enemy.
“Don't you remember,” she asked, amazed, “how you killed him and saved all our lives?”
“I forget them after I kill them,” he replied carelessly.
When she expressed a doubtful hope that Tinker Bell would be glad to see her he said, “Who is Tinker Bell?”
“O Peter,” she said, shocked; but even when she explained he could not remember.
“There are such a lot of them,” he said. “I expect she is no more.”
The novelty, purpose, and horror of peter pan is that he views the world as a child would toys. Everything is a game and, to him, has no permanence. If you asked someone about a made up fiction game where they killed their doll a year ago... well they would not remember and Peter is actually the same way.
He is one of those people that seem lovable and fun if you only know them for a short time. Other than that he is a monster and a tragic character in a lot of respects.
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