Not entirely sure on the first time I actually was alone, but my best guess would have been around the age of 7 or 8, living in a state that has 8 on the list of an appropriate age to be allowed to do stuff without a responsible adult or older sibling.
lol my parents made my 14 year old step sister watch me when I was 7, to which she never actually did. She'd run off with her friends or with another boy and leave me to fend for myself. I'd go up the street and play with this other kid and his brother at his parent's house. My parents eventually found out that she wasn't doing her job. I use to lie for her because I enjoyed the freedom.
Stay inside, lock the door, and don't answer the door for anyone. That was a common thing said back during the 90s and even early 00s. I don't recall when I was allowed to stay home by myself either, sadly.