Euthanasia?

I make no secret on this Blog of my extremely mixed feelings on the matter of 'mercy killing' and associated matters for human beings and am all too well aware of the difficulties found in any argument which attempts to 'draw lines in the sand'.
For example, if you choose not to intervene medically in the case of an 87 year old with a terminal disease - how easy is it morally to justify treating the burst appendix of a twenty year old? Just 'a question of scale' do you retort?
Fine. But at a point in between, precisely where do you intervene/not intervene?
Tricky innit?
A further complication - not too disconnected from this - came to me as I performed my early morning ablutions today.
Do we agree that God must choose the moment of each person's death?
Most believers will answer in the affirmative - so let us consider this:

If it is wrong to intervene to end a life as this may be deemed to be 'God's territory alone' - how do we ever justify trying to save a life that would otherwise be lost without equally straying into a 'God-defying' scenario?
I repeat - tricky innit?