I would guess they are old enough. Our kids get to wonder the campgrounds as long as they respect others sites, property and privacy. Our oldest just turned 16 this past winter. The other 2 are 14 and 13. If we leave them alone at the campground, they have strict orders to stay in the trailer and keep it locked. We have cell phones if they need to contact us. We've only had to do this a few times in the past few years. But I figure if the can bike around the camp or go to the pool/lake, they are responsible enough to handle an hour or so in the camper. They need to learn some responsibility and actually be able to prove they understand that responsibility.
If you trust them, they are old enough in my opinion.