What Is The Magic Age For Leaving Your Kids In The Car?

I don't think there is anything wrong with leaving a 15 year old in the car while you run into the gas station to grab a soda, but I do think it is wrong to leave your 4 year old in the car. Although, I have left my son in an un-running, locked car while I ran into my house to grab something. I have to admit that the whole time I was in my house, grabbing what ever it is I was grabbing, I was on pins and needles cause I was not 100% sure of what my son was doing.