I prefer to frame it more like this: "People in your age range drive less or cannot legally do so and as such have few other options for transit, so we'll give you a discount. Also, the negative externalities of rail travel are far lower than other modes of transit so we should encourage as many people as possible to use it. When a frequent young user of rail transit grows out of the lower price bracket, they will likely continue to use the service to help keep it affordable for all."