I can't remember the "age" but when DS was in 4th grade, I left him home alone to run DD to dance, maybe for 20 or 30 minutes and then worked up from there.
DD is 12 and is rarely home alone. She looks young for 12. I worry about if a stranger would come to the door or home invasion with her home alone. She is also my child who has set the house on fire and flooded her bath room.

She would never get into trouble, but I just worry more about her.
When we were in DC this past summer, one morning I had told DS to get up a million times. He didn't. Chloe and were ready for breakfast. I made the statement we were going to breakfast and then to the zoo, if he wanted to go, he better find us at the breakfast buffet. So I left his booty in the room. Twenty minutes later he was downstairs, showered and ready to go. He had no trouble getting up the rest of the trip. PS - I would have left his sorry BEHIND in the hotel room alone that day. I was so irritated with him. Sorry, it's a parent of a teenage boy thing. I am sure Sea understands where I'm coming from.
Around the age of 12, I think at WDW I sent Chris to the food court at Pop to get drink refills for us and things like that. He is very mature and big for his age. He always comes off as being older than he really is. I think around this age, we may have left him in the room alone as well.
I hope that helps!