The ethics of it all

So this summer I have two sessions: 6 weeks of pediatrics followed by 5 weeks of obstetrics. The last four weeks I've been at a rehabilitation clinic/sub-acute facility for children with brain injuries because our regular hospital fell through at the last minute (boo-hiss!).
I've gotten over the disappointment, but if nothing else, this clinical has made me think. We have several patients, about 25 who are completely bed-ridden. A few of them are able to track me with their eyes, but a good number of them are completely and utterly nonresponsive. There's this one pt who has been there for over a dozen years. She/He (protecting the innocent, here) choked on a fruit pit, a little one, when he/she was just three! This caused an anoxic brain injury, or lack of oxygen to the brain, and permanent brain damage. This patient now lives a life of spasticity, rigidity, blindness, unresponsiveness. There is absolutely zero quality of life. The nurses and aides try their best to make him/her comfortable, but the patient just lays in bed 24/7. We have to tape the eyes shut to keep them from drying out. He/she lives on a ventilator and has to be suctioned every 2-4 hours and fed through a gastrostomy tube every 4-6 hours.
So here's the ethics question: at what point is this person allowed to die? This isn't a euthanasia question as euthanasia is the use of artificial means to end life. This individual hasn't had any quality of life since three years of age. He/She went from being a normal toddler to a bed-ridden, mechanically ventilated individual. The parents visit only a few times a year. If it were me, I would rather die. I would much rather be rejoicing with my Heavenly Father than allowed to lie on a bed with no quality of life whatsoever.
The question that then begs to be asked is who makes the decision to pull the ventilator? And when? Is it after 6 months? a year? Should it be the parents who are guilt-ridden for "allowing" this to happen? The doctor? The state/government? The insurance company? money? I'm not happy with any of those options! We could potentially keep this person alive for years and years to come. An infection will eventually set in that isn't caught in time or is drug-resisitant - but that could be decades down the road.
So this has been a hard rotation in that respect. I love the verbal kids: the ones I can talk to; and I love the parents who come to visit on a somewhat regular basis. I just wish I could do more...