As someone else mentioned, it's likely Ether that gives them the long lifespans. The Fallen worship Prime Servitors as their creators and lifeblood and in turn the Servitors give them Ether, which described as "life sustaining" in lore. Each house has its own Prime Servitor and Archon Priest so logically, I'd say the symbiosis of Servitor/Fallen can make them almost immortal. Servitors were around during Whirlwind, and are described as beacons of hope when the Traveller left so they themselves are ancient and likely the reason why Fallen have lived so long without the Traveller. [spoiler]This stuff is on the Destinypedia, I'm not this clued up on lore, I just did some reading. 😅[/spoiler] With Smallen now existing, it's probably a Grogu situation where the baby in the trailer is already 70+ years old.