I'm not saying 9 is too young to use public transportation. That is an individual judgment call based on the area, the route and the child. But there is a difference between commercial air travel and public transportation. On most commercial flights you are usually required to pay a little extra for an unaccompanied minor, and they are given special attention and assistance by airline staff to make sure they get to their destination safely. There is no such assistance on public transportation (that I know of).