Saying that James M. Barrie was manic-depressive understates the case a bit.
Did you know that when you're subjected to stress, your body shuts down? Not entirely, of course - you can still breathe and walk, and your heart keeps beating, but everything unessential grinds to a halt so you can focus on your own survival - you stop digesting food; that's what makes your mouth dry. Your menstrual cycle (if you have one) gets thrown out of whack. You stop growing.
You stop growing.
It's an infinitesimally small amount of time this happens for, usually seconds or minutes. But under prolonged stress, years worth, particularly during the formative years, it can add up.
J.M. Barrie's ten-year-old brother David died horribly in a skating accident when James was six. David was their mother's favorite by a mile, and his death left her bedridden and delirious for a decade. James took care of her; tried to, anyway. Refusing to believe that David was dead, she clung to the idea that good little boys never die, never leave their mothers, never get to the point that they venture out on their own, and saw enough of David in James to render James responsible, in her eyes and eventually in his, for David's death.
Barrie never grew above five feet. He developed little in the way of secondary sexual characteristics apart from facial hair. His marriage remained entirely unconsummated.
Good little boys never grow up.
Neither did Peter Pan.
Bits and bobs taken from: http://en.wikipedia.org/wiki/J._M._Barrie
and from a story related by Robert Sapolsky in a 2005 episode of Radio Lab available here:
http://www.wnyc.org/shows/radiolab/episodes/2005/02/11