"Just" 20 minutes? I've always been on the "wrong" side of the "kids in cars" discussion on MDC (ie. I don't think it's automatically morally reprehensible), but I think 20 minutes is quite a long time. I've left my kids in the car many times, and much younger than 10. But, I've done so to return shopping carts, or while I'm unloading groceries at the house (with young kids, I feel they're safer in the car than they are running around between the car and house - and possibly into the parking lot - while I'm going in and out with bags).
I think set ages (eg. "under 10") are kind of ridiculous, personally. My oldest was more able to look after himself at six than my third one is at almost eight. DD1 would have been fine on her own for a few minutes (eg. five or less) when she was about five. (I never left her, because ds2 was with her, and he was too young.)
Someone here on MDC once commented that 14 was too young, because the threat of kidnapping still existed. I can assure anybody who's interested that ds1, at 14, was far more able to defend himself against a kidnapper than I was! (He'd reached almost six feet in height, was training as a gymnast, had some martial arts experience, etc. - I was very pregnant.)
It's not the age. It's the child. That's why the laws about this kind of annoy me. They assume that a legislator who isn't on the scene can do a better risk assessment than the parent who is on the scene. Sure - some parents are negligent. You can't legislate that away.