Klaus: I think Klaus would be pretty good-humored as a senior. He might feel like he doesn't need to be anywhere in particular, because his feet have done all the roving and wandering they've needed to, and all he needs now is a place to rest with people who love him. He'd like to think that all that going places and seeing things has maybe learned him a thing or two, and he'd love to share that with people. Or maybe just what he knows about the sea, and the stars, because people can always stand to learn about those things.
Andra: In a way she's already there. She is now at least two years past her natural lifespan, but she has the body she had when she was in her prime. Give it a few more years, and she'll be the Aetherborn equivalent of the Bicentennial Man. I think she's really enjoying it, but part of her is sad that soon, there won't be a single Aetherborn from her homeplane who was alive to know her.
Lys Ora: Would probably be really grumpy. Either her thirst for vengeance has never stopped, in which case she'll be a senior on your ass for every little thing, or it's burnt out and left her full of regret about things she can't take back, and things she hasn't done.
Vanya: I think Vanya would take to age pretty gracefully, gentleness is in her nature, so having an excuse to do nothing but sit around, maybe make some tea, dispense nuggets of wisdom, that might agree with her temperament very much.
Traenor: Senior Citizen Traenor...He'd probably take after Syr Caitlin, who was a knight well into her 70s and continue to kick it. Age never slowed his mentor, and he won't let it do it to him. He would have to reckon with maybe having less strength, the way Syr Caitlin did, but hey, maybe he could finally learn some more advanced magic like he'd always tried.