How does Earendil speak Beorian?

[Tar Elenion]

Originally Posted by Huinesoron

Elwing was about three, Earendil about ten (always "about", because Tolkien's timelines were in flux and quite old).

Where are you getting the ages from?

505 [> 503] Birth of Earendil Half-elven in Gondolin (Spring).

511 [> 509] The Second Kinslaying

512 [> 510] The Fall of Gondolin. Death of King Turgon.

503 Elwing the White daughter of Dior born in Ossiriand.
WotJ, ToY



And Annals in HoMe 5:

300 [500] Here was born Earendel the Bright, star of the Two Kindreds, unto Tuor and Idril in Gondolin. In this year was born also Elwing the White, fairest of all women save Lúthien, unto Dior son of Beren in Ossiriand.

Also noted in Shaping.