Regardless of whether or not my child is responsible or mature enough to be left alone at the campground or at home, the campground where we have a seasonal site has a rule that they must be 18 years old or older to be left alone. That does not mean they can't be off and playing or riding, but we have to be on site. We don't want to get kicked out, so we follow the rule. I would imagine it's a liability issue for them. Most campgrounds we have been at have also had this same rule. It's probably in the fine print.