I agree with those who say that we have to begin teaching independence, and so the reply may depend on the child's previous experience rather than age.
If she has never travelled alone, rarely gone on a train etc,. then she needs some experience and preparation. I made sure that all of our kids got such experience. For instance, on an outing, a child would pay for their own ticket, sit alone etc. knowing that the rest of us were on the bus / train if needed.
They began with short journeys where someone would meet them. I recently saw my 10 year old GC on to a bus, then went to meet him off, just for the experience.
I think that learning to be independent, watch out for yourself and so on, are important skills. Young people are more likely to run into trouble if they don't know how to handle themselves, or spot trouble and know how to avoid it.