I think part of the inherent horror of the train is that it truly does not care the age of its passengers. It will literally take anyone.
We see lots of teens, of course, like Jesse and Tulip
We see young adults like Ryan and Min
But we also see the show repeatedly hammer in the fact that the train isn’t afraid to take extremely young children, children who could (and do!) spend the bulk of their formative years on the train. Which is genuinely horrific if you think about it more than 5 seconds. These kids likely have families desperately searching for them and are going to be forever altered by having some of their most critical years on the train. Book 3 perfectly shows how terribly wrong that can go.
But it doesn’t even stop there!! We see snippets that the train will take parents and the elderly too. Like I wouldn't be surprised if some of the on train deaths were old age or more "natural" causes than something like Simon's death.
I just think part of what makes the train so inherently terrifying is how little it matters how old you are, what you're doing, who you are. If the train deems you need to get it together, it'll come for you regardless.
