Question: Shouldn't the lung Kes donated to Neelix (after his were stolen by the Vidiians) age rapidly like her or would it adjust to his lifespan?

VOY s3e12 "Macrocosm"

Not a captain, nor a doctor, but I would imagine that tissue would age based on the body that contains it due to whatever chemical processes the body uses to make aging happen.

Did they address anything about Neelix’s body rejecting the tissue from Kes? Or is it just assumed their level of technology overcame that little detail? Or does Neelix just secretly take anti-rejection meds now?

Total assumption. Like O'Brien's PTSD, it's never brought up again.

Well, we know Ocampa were capable of living longer thanks to that other group with susperia (was that her name?). Maybe they just died of cellular boredom what with being underground the whole time without a regeneration and entertainment chamber. Maybe space adventures were more fun so he stayed alive?

Yeah, they covered jt. IIRC federation medical tech couldn’t do a transplant from any crew member but the Vidiians altered the immunogenicity of… something and to them anyone in the room was compatible for donation.

Neelix just collapsed one day, because his lung ascended into energy to be with Suspiria