The answer to the question depends both on the child and the circumstances, as the OP implies.
Are there trustworthy adults like neighbors in the vicinity if there is an emergency or a disaster, or is the child really alone?
Is the child the sort who could handle something unexpected? Is the child the sort who is generally trustworthy?
Is the child the sort who is happy to be alone or does the child get unduly anxious when alone?
Is the child going to be left in a safe neighborhood?
Is leaving the child home alone going to be a routine?
Is the child going to need to prepare any meals? If so, is the child competent at that?
Will the child be able to contact someone if they have questions? Will there be any siblings around?
Will friends be likely to drop by? How is that going to work?
And so on.
Having said that, the law has to be considered, and that varies from one jurisdiction to another, and usually even then there is an amount of time that a child who is legally old enough to be left alone can be left attended. That is the kind of thing you can call the local police department about. In our area, they said it was permissible to leave a ten-year old home alone for several hours in a secure place where the child could meet his or her own needs. Even if your child is totally capable of being left alone, you have to stay in the boundaries of what is legal.
In your case, OP, keep in mind that your parents were able to see how it was working out with you. If you seemed to tolerate it well, it was legal. If you had not tolerated it well or if they hadn’t prepared you for it by the expectations placed on you in the years prior to being left alone, they wouldn’t have been able to do it, even back then. Mileage does vary.
If you found that you didn’t tolerate it as well as your parents thought you did, that is something to consider before you leave your own child alone. As far as whether your parents were doing the right thing, you’d have to ask yourself about the situation and whether they exercised care to make certain everything was OK with you or if they did have a choice about the situation.
Children can be left home at any age for it is God who watches over them.
If the child comes to harm it is through their own sin and not even your presence would prevent that.
I will pray for you.
What? That’s supposed to pass for a Lasallian answer?
Be serious, this is a serious question.