I’d argue that most residents would likely have access to better offers locally. As for the age limit, I think it just lets you get a cheaper inter-rail, which is subsidized by the DiscoverEU. AFAIK, you can always buy a full interrail pass for all countries. And yes, non eu people can buy eurail.: but that one is much more expensive than the equivalent interrail, I think. Maybe I’m wrong about the last one tho