James himself never grew up, actually. Probably because of the trauma, he developed stress dwarfism, and only grew to be about 5 feet tall (although there are conflicting reports - some say he was much smaller).
Barrie never said that Peter and the Lost Boys were, in fact, dead, and maybe he didn't really mean it that way. Peter, the story said, was in his baby carriage, heard the grown-ups making plans for him when he grew up, and he wanted none of it, so he ran off to Neverland. The Lost Boys were children who had gone missing and ended up in Neverland.
Imagine the milquetoast movie "Finding Neverland" if it had told the real story. Imagine if, instead of a moping post-Gilbert Grape Johnny Depp, it had starred Emanuel Lewis or Vern Troyer. That would be preposterous, of course. People with disabilities do not play romantic leads. People with disabilities are either comically lovable or heart-wrenchingly tear-jerkingly Courageous. They certainly don't have clandestine affairs or successful careers. Who'd believe that?
One thing you can say; Peter Pan is probably the most beautiful work of art ever to come out of survivors' guilt. Although Stephen King's "The Body" is up there as well. More on that later, perhaps.