So let’s assume this is a legitimate post and this GParent is concerned.
It’s not the age, but the sense and character of the child being left alone.
At 12 many “children” are quite responsible and neither want or need a childminder.
However, it’s a long day, but I’m sure parents keep in touch and maybe she has a friend round and I’m assuming she has a mobile phone.
I guess if she is ok with it and her parents are too, you just have to understand it’s their child and their rules.
Not sure what else to add, it’s not unreasonable to care, but it’s not really your concern.
So long as you still see her and can contact her and she seems safe and happy, I guess job done!