The Darkling was born many centuries ago to Baghra and an unknown Heartrender. He was raised by Baghra and never knew his father.

How is the Darkling so old?

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. ... 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.

Does the Darkling actually like Alina?

So yes, it seems like the Darkling really does care about Alina, but Bardugo pointed out that she didn't want to make her antagonist one-note because in real-life, villains rarely announce themselves as such: “The Darkling is probably the most, maybe second to Kaz, the most popular character I've written.