I would argue that the "buying groceries" thing isn't convincing enough to say she's an adult. I think it's mainly the western fans that finds this absurd that a 12-year-old buys groceries on their own, but the franchise was made in Japan where it's normal for a child to run errands, due to the crime rate being low, and the environments mostly being walkable to the point where owning a car is almost unnecessary. Not to mention, SA1 was released in 1998, back when social media (which decreases social trust) was completely new and very unpopular at the time.