HoldMe, I think age is up to you, we all came from different dimensions anyway, right? This means different time frames in different histories in different worlds.
Rinne for one, is generally old from her dimension of an extremely slow aging species. But with the physical appearance of a youngish human woman.
Thomas, aside from the ability to change her appearance, she can duplicate a person entirely by following their appearance. When she adapts another person's appearance, she can gain access to that person's thoughts, abilities. skills and personality. However, she can't mask her own scent nor "presence" like an aura of sorts. Her life energy. And she can't really do that if the person is far more powerful than she is, her body can't take it. And she has to have eye contact with this person while she copies them otherwise she'll only get their physical appearance and nothing else. Yeah. Uh. Uselessly useful, huh?
That aside, I think First Gen should have the ability to open and close portals and the ability to connect with the Sister (there should be a name for her but the Sister would do as of now) in spirit, meaning, when they enter a Plane, the Sister lets them know what that Plane is like in glimpses. So, FirstGens who Walk into a certain plane for the first time will know what species live there, what kind of life the species live, ect. but nothing too specific. FirstGens just seem to know that much, but most is from their experience.