My 17-year old daughter usually goes with me, but it's her choice. She doesn't like to hang out alone and she doesn't have many friends that want to go camping with us. To be honest, when we're camping, we go to hang with the family and get away from daily life, so we don't expect to leave her alone.
Otherwise, we have left her alone when she was sleeping/napping, but I usually don't leave the park and the dogs keep her company. And do I worry about her? She shoots much better than I ever will, so not really!