How young is too young?

How young is too young to leave a child home by themselves?

Here's my dilemma.  My oldest, who is almost 10, was sick.  He was throwing up all day.  I had to go pick up my other son from school.  I told him and his youngest brother, (who is also home sick) to get ready to go pick up their brother.  They walked around the house miserably sick, I grabbed a throw-up bucket and their shoes and they got in the car.  We had to make a few stops for some sickness then went on our way.  My son asked me why I couldn't leave him home alone while I made this trip.  It got me thinking,  How old should he be before leaving him?  I know it is more of a maturity thing as well as age and I told him probably middle school.  This is a 30 minute round trip and I know he would have been okay if I left him but the thought still made me nervous.

So, here is my question.  If you have left your child home alone, how old were they?  If you have not, when do you think you would?