Worth considering how car-centric America is. An 8-year-old is unlikely to have access to a grocery store to which they can independently travel. Once they're at the store with a parent, they'll just travel the aisles together. It's not as though many young children have the funds to make purchasing decisions, anyway.
Note that they're talking about an aisle alone, not about going to the grocery store alone. My kids were older than 8 the first time they walked to the grocery store by themselves, but I'm confident that I told them "you go grab the peanut butter you like" while I was grabbing something in a different aisle when they were around kindergarten age.
Can't speak for others, but my 3yr old has definitely gone done an aisle alone. Much to my dismay, but alas, it did happen. These are the things that happen when you don't put your kid in carts and strollers all the time.
It also has the added benefit that he interacts with a lot of strangers while we're about 10ft away. It's good for him to learn that people are usually good.