gosh, I can't even imagine my kids not wanting to go into the store with me. In fact, as soon as we pull up somewhere, all four are fu-reaking out b/c they want to come inside with me - and I have to choose who to leave (with DH) in the car and who to take in...or if I'm in a mood, I leave them all and let him deal with their craziness.
No, but seriously, I dunno. Maybe 12-13? My older two kids are 9 and 7 and even if I could talk them into it, I don't think I'd feel comfy leaving either one of them while I go shopping. If it were a quick in and out thing like gas/library books/dry cleaners/pizza pick-up - then maybe.