I'm not sure if there is a legal age or not, but I know I wouldn't leave my kids home alone unless I was comfortable about it, and knew that they were responsible enough to look after themselves without supervision, or if there was an emergency/crisis. I'd be guessing not until they are 13 or 14 or so, depending on the child.

I don't think you were being negative about it!! I think it was a responsible decision, considering you mentioned he doesn't know the area very well (therefore probably doesn't know the people in the area very well) or the address/phone number. I definitely don't think your MIL should have said that to you, after all it's about what you are comfortable with, and can you handle the consequences of those decisions.