Ah, interesting thread as I have a 13 year old ds and a 7 year old ds who will be soon be 8. The 13 years of never having a family member to babysit, apart from dh, should soon be drawing to an end.
To be honest, using my own personal judgement, I would feel fine about my oldest son looking after my youngest for an hour or so in the evenings if dh and I were just round the corner. Even more so if ds2 was in bed. As long as both boys were happy to loll around and watch a dvd (this would depend on circumstances and mood), I'd happily leave them. We have lots of friendly neighbours as well.
We have a neighbourhood pub within 5 mintues walk of the house. Night after night I am here with both boys, while dh wanders off for an inpromptu drink or two in the company of our mutual friends there. I am the only woman in this circle of friends who has children of my sons' age. The rest are childless or have older children so have no babysitting commitments.
I cannot join them much as I'd like to unless dh comes home or we pay a babysitter.
Although I would feel fine about leaving ds1 in charge, AFAIK legally he is still just that bit too young to babysit. He needs to be 14 years old I am told. As all our pub friends know ds1 is 13, and many of them are teachers so very up on childcare issues, I am trapped!
And I can't tell you how I inwardly seeth when one of them (a teacher with loads of extended family to help out) tells me (as she is prone to do at the very mention of my son babysitting)that she would never have left her oldest dd in charge of the younger ones - never ever did this, even when the oldest was 18 years old. She thinks it's unfair to heap responsiblity on the oldest. All so well and good if you have many other free babysitting options, as she did....