I leave my 15 yo now, and leave my 12yo (although for no more than 1/2 an hour)
I wouldn't leave them together - they squabble and would end up arguing.
Neither would i leave them (either of them) looking after my 10, 6 or 4 yo.
I will not leave any of the younger 3 at all - even for 5 mins.
I don't thinkn it's fair for them to have to take responsibility for the younger ones - it's an unfair burden.
The main reason that i don't leave them is it does not feel "right" - so i don't do it!
If you can leave them, and not have a horrible knotty feeling in your stomach, or be at all concerned then fine..... I guess each of us are different on this......
In response to the op, I think YANBU at all...... I turned up at my sil to drop off a birthday card and hy neice had been left on her own (8) I waited for a while - apparently sil had been gone a while already, but when she didn't return I texted sil and took dneice with me - I felt wrong about leaving her. Now I am sure a lot of you would say it wasn't my decision to make, but I couldn't live with myself knowing I had left her if anything had happened to her. 
Why do some people have kids? 