I know this is going to change the mood somewhat but I really don't think that under 16, or actually 18 is a good idea.
I had a set up that sounds identical to Alicerose, in terms of helping several local families from the age of 12, and it all seemed perfectly simple (why don't I feel like that about babies now?!)... but then a few months before I turned 15 I was looking after a very new baby (3 months), and his elder brother, when the youngest died of SIDS.
Neither of the parents was as contactable as they would be now, because we have mobiles, but it was the single most traumatic thing ever to have happened to me and came back to haunt me many years later in the middle of a horrible labour and delivery. I was the original child born middle-aged, so my parents (and everyone else's) thought I was an angel, and so mature... but nothing, nothing prepared me for that.
I'm sorry, I realise this isn't exactly what you were looking for, but if you think of a worst-case scenario, would you either a - want a 14 year old in charge of it, or want to put them in that position.
Obviously my experiences colour the kind of help we get (my brother, a 26 year old au pair, that's pretty much it), but it feels impt. to show what can happen (how ever much you don't want to think about it).