To know Jumin's age in Another Story, you'd likely have to comb through the story carefully. Sometimes it's explicitly stated, or you might have to infer it from context clues.
Well, in the Casual Story, it is possible to date Jumin. The story provides opportunities for the player to get closer to him through different choices. For example, by choosing to participate in certain events he is involved in or having conversations with him that show an interest in his life and thoughts. These interactions gradually build a connection and can lead to a dating relationship with Jumin.
I'm not entirely sure as I haven't read it specifically. But generally, a visual novel about two characters like Jumin and Zen might be about their relationship, their individual stories, and the events that unfold in the world they are in.
Well, not always easily. The casual story has a lot of branching paths. If you follow the paths that are more likely to lead to Jumin - like certain social event options - you might meet him sooner rather than later. But if you go off on other tangents, it could be a while before you encounter him.