Is a 5 year old to young to stay alone in the car for 20 mins?

I went into the market and came out with my kids, and when we were returning the cart the car right next to the return station had this little boy seriously 5 years old waving to me. I looked in the car and he was alone. So I packed my kids in my van and drove the van and parked right behind the car and waited for the parent. 10 mins go by and ... Read Morestill nothing and the boy is sitting in the car staring at me. So go hunting for my cellphone trying to rememeber where it went in the van. another 5 mins go by I finially find my cell phone and call information for the local police depatment. After fighting with dum 411, I finially have the number and am writing it down. When low and behold the dad finially arrives after almost 20 mins. I hang up my phone and get out to confront him. I tell him that I was just in the process of calling the police and why would you leave a little child in the car alone. He yells and says "lad y I was only 5 mins" but since I had been parked behind the car for almost 20 mins I confronted him with that. Then he says the door was locked ...I laugh and start dicussing my opinion in which he backs down and says "whoa your tough and I can actually see your point, I didn't think it was that bad till now" also says "Thank you that people still care out there" shook my hand, I got say hi to the little boy..Jeff and then was off. But made me think is it common people leave 5 year olds in the car alone?