In Birthright, elves are immortal, like stated in Rulebook, page 7:
Cerilian elves are creatures of faerie dust and starlight, gifted with immortality and powers of mind and body beyond those of humanity.
They are only mortal in the Shadow World, as stated in Bloodspawn, page 27:
Unless slain, seelie faeries are immortal in the Shadow World but mortal in Cerilia. Conversely, elves are immortal in Cerilia but mortal in the Shadow World.
But nothing is said about half-elves in Rulebook beyond their resistance of 30% against aging attacks (elves have total immunity) and the elves treating them as Sidhelien (cerilian name of the race) and welcoming them fully in elven society.
However, in High Mage Aelies (a blooded half-elf mage npc) description in The Book of Magecraft, page 33, is said:
Aelies is rumored to be ancient - hundreds upon hundreds of years old. If such histories are true, his age is the only part of the High Mage that reflects his elven heritage.
He don't even has the Long Life blood ability to incite any error.
As a counter-point argument, in Blood Enemies book (page 22), Danita Kusor, a half-elf of vos heritage is cited in documents of Daznig (a sage) as a wizard searching for immortality (as she told in a interview) before changing in the awnshegh know as Chimaera. (She could be lying or the sage could be lying to not really risk his life in a interview with a awnshegh - but is a strange situation, since she can be recognized as a half-elf when not in monster form)
So, are half-elves immortal in Cerilia?