While I'm aware mutant genetics aren't so linear, is it possible that since one of his parents is immortal and one has retarded aging, that Kurt might have an extended lifespan?

Yes, and that's a very interesting point!

Although, in most future realities he's shown to age at a similar rate to the other characters, e.g. X-Men: The End.

@BlackWind: I am sure anything is possible but highly unlikely

My assumption that maybe it just slows his aging a little or maybe he will live an extra ten years(if it wasn't for the fact that he already died lol) I'd assume any slowed aging or extended life would be mild to barely any improvement

@John Valentine: Never said he'd look young though, lol.

@blackwind: you have a good point and based on your logic I agree he should naturally live long long time.

Yeah, being a Neyaphem should halt his aging. Being half should grant him at least a longer lifespan.