Did Elladan and Elrohir leave Middle-earth or stay?

[Olmer]

I think that emphasis should be put on the words "they should live even as Elves so long as their father remained in Middle-earth" The choice is not indefinite. They stay Elves while Elrond stays with them. They have to make a decision at the time of his departure - to go with him and become 100% Elves, or to stay behind and accept the "gift of God for the Mankind".
The same with their sister Arwen, she lost her immortality with her father departure. Considering her elven heritage, she could live a quite long life, in comparison with ordinary humans lives, but not indefinite, just like her late husband, whose ancestors lived 3 times longer than he was. I guess she became weary of the purposeless life, after all she was living for 2600 years sitting in the woods and needlepointing. After a relatively brief by her standards exiting period of being a wife of the King and a hostess of the King's Castle, she returned back to the square one: woods and needlepoint. You could hang yourself from such a perspective on the rest of your life.