From the Peter Pan Wiki: There's also a dark playful side to Peter, that's only hinted at in the book. Barrie stated in the novel that Peter would "thin the Lost Boys" out when they got too old or when there was too many of them (meaning he may exile or kill them), and sometimes in battles against pirates Peter would switch sides just for the fun of it. It's also stated in the book that Peter would go out and "hunt pirates" for fun.
Tl;Dr: Pan would "thin the Lost Boys" when they got too old or numerous.
Tl;Dr: Pan would "thin the Lost Boys" when they got too old or numerous.