I expect anyone over the age of 12 or 14 or so to be able to take a train by themselves.
I'm assuming you mean someone in a place where trains/subways are common. Because I can remember the first time I was in a city that had one, and I sure didn't feel comfortable doing it alone. And I was well over the age of 12 or 14. Some people don't grow up with that experience and don't have the slightest clue what to do - even on what a resident feels is "so simple" a trip or one that seems to just be a mere Point A to Point B thing.*
Heck, I needed help just getting a ticket and then getting on the thing. I wasn't familiar with any of the steps involved with it.
* I realize my comment may not apply to the OP's situation, as it could be that both she and Mary are in a place where Mary has encountered the trains before. I'm just pointing out that the generic "everyone over X age should be able to do it" mentality isn't, necessarily, correct.
No I don't only mean someone from a place where subways/trains/public transit in general are common. I mean any normally intelligent person over the age of 12-14 or so.
But please don't forget I went on in my post to say I think all the details should be gone over in advance - what to look for in a sign, how to read the signs (because honestly someone might not know what a hooked arrow means, or abbreviations, or whatever quirks that systems signs have), going over the map, what to expect, etc.
Its like anything else someone does for the very first time ever - go to school, grocery shop, job interview, sporting event - if they are given good instructions and an understanding of what to expect and how to navigate it, they should be able to do it. No one should be thrown into a situation blind, but I do expect a person to be able to figure out "go to train station/buy ticket/find train/get on train/go one stop/get off train"
so long as things like "this is how to buy a ticket"
and "this is how to determine correct train"
are known/explained in advance.
I would expect a rural person to be able to explain something similar to me and expect me to do it too. Remember this isn't a dangerous thing - its not "perform open heart surgery and if you mess up someone might die
", its more a "worst case scenario you end up lost and have to take a very expensive cab home
" situation. Does the worst case suck? Yeah, but its survivable. And its unlikely. Chances are, well prepared in advance, the person will find their way home on the train. Maybe a few minutes of disoriented, maybe a missed train and a 20 minute delay, maybe feeling a bit foolish for a minute, but ultimately most people won't hit the not-so-terrible-anyway worst case scenario.