Can a person under 18 travel alone?

Yes, but you need to contact each airline for there specific policies

Each airline has their own standard concerning what an Unaccompanied Minor is, so please be sure to check with the specific airline. For example, Southwest allows minors ages 12 - 17 to travel unaccompanied.

Yes. Valid boarding pass

Yes. It will cost extra for an escort which is required for an unaccompanied minor.

Yes