Surely, these days it is so much easier for anyone of any age to travel. Mobile phones mean they can be in constant touch, and seek help if required.
Few years back I used to take my (then) young teenage g.daughter after school to her Music School. a bus ride away in East London. Always talked her through that bus journey, so she knew where she was going. When I was unwell one week, she did this on her own. Unfortunately, just that day, an incident meant that the bus was diverted, so she had to get off quite early. However, she phoned her Mum, and her Mum was able to talk her through walking the rest of the way.
Many, many years ago, my young teenage twins used to take themselves right across London to go to Crystal Palace to attend their diving training. They were both small for their age (though as athletes, deceptively strong). One time, as they was sitting chatting on the underground, one of them had their ticket in her hand and a couple of youths pushed past, grabbibg that ticket from her. They (youths) rushed down the tube, my daughter rushed after them, caught up with them - they were very much bigger than her. grabbed the one still holding HER ticket, kicked him in the 'you know where' and calmly, took her ticket back, and then sat down happily. WHen his mate came over to her, she grinned at him, offering to do the same to him. He retreated.
Do not think it is any more dangerous now than then to travel, she always put her ticket away after that. I would have loved them to have had mobile phones back then. One time, they were travelling back across London (South West to NE London) late at night, and there was a major railway incident which meant there were no trains. They did phone me (public phone box), to let me know - but said they were trying to find another way to get across the river, so I was unable to tell them to remain in one place for me to drive to collect them. Did have me concerned for the next couple of hours until they phoned to say they were nearly home, and could I collect them.
TBH, I find that the helicopting parenting of so many teenagers make me far more concerned about those youngsters learning to cope. They are so very un=streetwise because they are always taken and collected from everywhere they go.