[symestreem]

Luthien was also mortal when she had Dior, but Dior was considered an elf and Thingol's heir. Let's see: Elrond is 5/8 elven. He marries Celebrian, who is 8/8 elven. Their children will be 13/16 elven, although they will carry all the traits of elves and will be considered elves. Arwen marries Aragorn, who is 0/16 elven. He may have elvish blood, but he is not elven. 13/16 + 0/16 = 13/16,/2= ...ehh... 13/32 I believe. So, Eldarion is 13/32 elvish and 19/32 mortal. (Don't try and follow my math...) Also, not all other children of half-elven unions were considered mortal. Dior and Earendil were the only two true half-elven, and they were both considered elves. There is no record of Dior choosing his kindred, although Earendil did so. Elwing was the child of a half-elven and an elf, and she had the choice to belong to either kindred. Elrond and Elros were born before their parents chose their kindreds, which may have been why they also had to choose, although Earendil and Elwing seem to have been considered elvish anyway- they led the remnant of the elves from Doriath and Gondolin.
Sorry if this is a bit rambling and unrelated to Eldarion. Eldarion would probably end up like the men of Dol Amroth- mortal with elvish blood. Although why he wouldn't be considered elvish like Dior I don't know- the circumstances are quite similar. *scratches head* Oh well.