My spouse is 16 years younger than I am (I'm 50; she's 34). Nobody can tell, because I look younger. I know lots of people say that, but I can only tell you my experience. So, people are often shocked when they find out, but because we don't LOOK different in age, they get over it very quickly. I do wonder if at some point, age will catch up with me and it will become more obvious. In the meantime, lots of her partners are her age and thus more sexually...ummm...available? Something something stamina? IDK. But I don't get jealous of it because I'm the age that I am, and I also have other partners. Besides, that's only one part of our lives. We are in synch in most ways. It's funny when she doesn't know certain bands/actors, but honestly, she's not great at that in general.
I think one of the perks of poly is that you can experience relationships with all sorts of different people. Age is just another difference to embrace!