It certainly wasn’t invincibility as the Night Elves who died before Nordrassil was detonated can demonstrate.
It was a simply a suspension of the aging process once maturity was attained.
But again in the Crapsack World of Azeroth, the last thing a hero needs to worry about is dying of old age.
The Night Elves turned into Satyr though by the corruption of Xavius are still immortal as they’ve become demons.