Medicinal Advantage or Merely Sustaining Life?

What popped into my mind today was Medicine. When do medical advancements stop being an advantage and start being a way for parents / family / loved ones to prolong a life.
I was talking to a friend of mine and she was a bit down because her 3yr old neighbour most probably has leukaemia - and she already has down's syndrome, and has had about 4 operations on her heart. And my first thought? If it wasn't for these so called medical advancements being an 'advantage in life' this girl would've died closely after birth. This girl wouldn't have had to suffer an immense amount of pain, fuss and stress before she had even turned four because her parents had decided that everything needed be done for this little girl to be kept alive. Is that fair?
The child must have gone through unspeakable pain already; recovering from heart operations, dealing with downs (I don't know what comes with Down's syndrome) and now going through Chemo and bone marrow transplants?
A young life - a  baby - can't decide for itself. It isn't counscious in its decision making and it isn't fully aware of the surrounding around them. They don't think, they don't know what's going on and generally they can't speak until they're a little bit older (2/3ish) - so the grown ups decide. The doctors have taken an oath to ensure that they do everything in their might to save the life of a person, but at what point does this become too much? At what point does it become almost unethical to keep the child alive?