You have to be to cross into How old do you have to be to cross from Canada into the U.S. by yourself?

My daughter is 17 and has a valid pport. She would like to come and visit me here in New York. Can she take a shuttle with a notarized letter from her father (and myself) all by herself. She would be catching a shuttle in Vancouver to take her directly to the Seattle airport. Or, possibly taking a train from Vancouver to Syracuse and flying back. I can't find this information anywhere.