In Birthright, elves are immortal, as stated in Rulebook, page 7:
The Cerilian Elves are creatures of fairy dust and shining stars, endowed with
immortality and powers of mind and body exceeding those of humanity.
They are only mortal in the World of Shadows, as reported in Bloodspawn, page 27:
Unless killed, the seelie fairies are immortal in the world of shadows but
mortal in Cerilia. Conversely, the elves are immortal in Cerilia but
mortal in the world of shadows.
But there is no evidence that half-elves in the rulebook resist a 30% resistance against aging attacks (elves enjoy complete immunity) and treat them as Sidhelian (race name) ) and welcome them fully into elvish society.
However, in High Mage Aelies (a half-elf bloody mage swim) in The Book of Magecraft, page 33, it says:
Rumor has it that Aelies is old – hundreds and hundreds of years old.
If such stories are true, his age is the only part of the High Mage
it reflects his elven heritage.
It does not even have the blood capacity of Long Life to incite any mistakes.
So, are the half-elves immortal in Cerilia?