Many seem to be confused about the fates of Elladan and Elrohir – I know I was for a long time. (Bold is my emphasis)
We know Elladan and Elrohir stay in Middle-earth after Elrond leaves:
There, though Elrond had departed, his sons long remained, together with some of the High-elven folk. It is said that Celeborn went to dwell there after the departure of Galadriel; but there is no record of the day when at last he sought the Grey Havens, and with him went the last living memory of the Elder Days in Middle-earth. - Lord of the Rings, Prologue
But after the passing of Galadriel in a few years Celeborn grew weary of his realm and went to Imladris to dwell with the sons of Elrond. - Lord of the Rings, Appendix B
So, do Elladan and Elrohir become mortal?
Most of the confusion stems from Letter #153, which was written in September 1954; after Fellowship was published, but before Two Towers. It says: “The end of his sons, Elladan and Elrohir, is not told: they delay their choice, and remain for a while.”
However, this is contradicted in Return of the King; though a lot of fans quote the letter.
Elrond chose to be of Elven-kind, and became a master of wisdom. To him therefore was granted the same grace as to those of the High Elves that still lingered in Middle-earth: that when weary at last of the mortal lands they could take ship from the Grey Havens and pass into the Uttermost West; and this grace continued after the change of the world. But to the children of Elrond a choice was also appointed: to pass with him from the circles of the world; or if they remained to become mortal and die in Middle-earth. For Elrond, therefore, all chances of the War of the Ring were fraught with sorrow. - Lord of the Rings, Appendix A
The question raised by many fans, is 'can “pass with him” refer to passing at a later time, after Elrond did?' Many think yes, but that is contradicted by the text. While the phrase “pass with him” can be interpreted that way, the problem with that answer (yes), is the word 'remained'. Remain means “continue to exist, especially after other similar or related people or things have ceased to exist.” It is very specific, time wise.
This is made explicit by Arwen, and a conversation between Elrond and Aragorn:
But the Queen Arwen said: “A gift I will give you. For I am the daughter of Elrond. I shall not go with him now when he departs to the Havens; for mine is the choice of Lúthien, and as she so have I chosen, both the sweet and the bitter. But in my stead you shall go, Ring-bearer, when the time comes, and if you then desire it. If your hurts grieve you still and the memory of your burden is heavy, then you may pass into the West, until all your wounds and weariness are healed. But wear this now in memory of Elfstone and Evenstar with whom your life has been woven!” - Lord of the Rings, Many Partingsand:
“What is that doom?” said Aragorn.
“That so long as I abide here, she shall live with the youth of the Eldar,” answered Elrond, “and when I depart, she shall go with me, if she so chooses.”
“I see,” said Aragorn, “that I have turned my eyes to a treasure no less dear than the treasure of Thingol that Beren once desired. Such is my fate.” Then suddenly the foresight of his kindred came to him, and he said: “But lo! Master Elrond, the years of your abiding run short at last, and the choice must soon be laid on your children, to part either with you or with Middle-earth.” - Lord of the Rings, Appendix A
It is made clear by both passages that Arwen/Elladan/Elrohir must sail at the same time Elrond does, or they become mortal. This cannot be denied.
My heart breaks for Elrond – he loses all three of his children to mortality, just like he lost his twin.