My metamour is much older than me, and I'm not sure how to relate to her

Advice on how to relate with a much-older metamour? I am 25, and I have just started seeing someone who is 35 and whose live-in partner is 42. He has other partners as well who are closer to me in age but none that he lives with. I want to be able to cultivate a good relationship with my metamour but I am not sure how to do that when she is closer in age to my parents than me! We have shared interests, and we all feel very positively about each other form what I can tell. No jealousy barriers.

There's a fine line between "being prepared" and "searching out trouble." If it's not an issue right now, try to relax! Focus on your shared interests and your shared partner. Don't make things awkward by bringing up the age difference or pointing out that the metamour is closer in age to your parents than you. 

Don't "fake it." If she brings up a band, television show, or other element of pop culture that's unfamiliar to you, just say you're not familiar with it and let the conversation move on. Be open to sharing - if she lends you a book, read it; if she recommends a movie, watch it. Same on the other side: be willing to share your world with her and be patient and gracious when there's a gap in her knowledge. In general, just be cool!

You two don't need to be best friends, and it's fair to expect that with the generational difference, there are some ways in which you won't be able to relate to each other. But you can be friendly even if you're not bets friends! If things are going smoothly now, and your mutual partner hasn't expressed any concerns about you coming off as aloof or immature or whatever, try to relax and just let this relationship be what it needs to be.