The Dior Dilemma

[Lalaith]

All Peredhil would've been mortal until Earendil and Elwing.

But hadn't the Peredhil all lived immortal lives even before this? Dior's dates are uncertain, according to the encyclopaedia of Arda, and he clearly was slain when young.
But Earendil and Elwing were both born around 500 FA. The voyage was around 573, was it not? They don't strike me as a couple of old dears - Elwing flying around in bird's shape and so on...so they clearly did not age like mortal men and women did.

And does no-one have the answer to my query about the twins and the Nandor?