Warning! Spoilers ahead for Shadow and Bone season 1.
Shadow and Bone hid a major twist in plain sight with the Darkling, who has lived for quite some time and is now known to all as General Kirigan. However, Kirigan isn’t the only name the Darkling has taken since he’s been alive. As season 1 of Shadow and Bone established, the Black Heretic has seen and survived a lot, rivaling a lifespan that would make most fictional vampires quite jealous.
By the time fans meet Kirigan in Shadow and Bone, he’s the general of the king’s Second Army, which consists of Grisha, in East Ravka. He built the Little Palace as refuge for Grisha and trained them to fight as soldiers in the ongoing conflict between Ravka and its neighbors to the north and south, Fjerda and Shu Han, respectively. Kirigan bemoans the creation of the Shadow Fold, a swath of darkness separating one side of Ravka from the other, as the mistake of his ancestor, one he must fix with the help of Alina’s Sun Summoner powers to better the lives of Grisha. What Alina didn’t know was the Darkling, whose real name was Aleksander, created the Fold. Since then, he's faked his death and changed his name many times to cover up his true identity and actions.
Shadow and Bone doesn’t provide the Darkling’s exact age, though he passes as a little over 100 years old to others in the present day. A picture of him in Baghra's cave reveals the man hasn't physically aged a day in centuries. The Shadow Fold was created roughly 400 years prior to the events of the series and the Darkling was already an adult by then. An estimate would put him at 500 years of age, if not a bit older. In season 1’s episode five, he’s somewhat frustrated about Luda’s mortality. After all, Grisha can still die and it seems he and his mother, Baghra, are the only ones who have survived for as long as they have. What’s more, Baghra tells Alina the Darkling is eternal, which suggests he could go on living forever.
As the Shadow Summoner, the Darkling is one of the most powerful (if not the most powerful) Grisha to have ever lived. His great power is what allows him to live as long as he does — the more he uses his Grisha abilities, which can rejuvenate and strengthen the body, the longer his lifespan becomes. The Darkling’s longevity might also tie to his grandfather, Ilya Morozova, who was powerful enough to dabble in merzost, dark magic that led to the creation of the stag, which enhances Grisha abilities, and the Fold. Merzost drains the life of any Grisha who uses it, but their regular abilities, which allows them to manipulate matter on a molecular level (aka, Small Science) makes them stronger.
The Darkling is already immensely powerful and has now had hundreds of years to practice and perfect his gifts, thus granting him an incredibly long life. He may not live forever, however, especially since the end of Shadow and Bone season 1 saw him using merzost once more to create new creatures, called nichevo'ya, that could follow him out of the Fold. The effects of merzost can clearly be seen on the Darkling’s face and it’ll be interesting to see if his continued use of magic will impact his lifespan or cut it short.