He presented himself as around 120 while claiming to be the great-great-great-grandson of the Black Heretic. He is at least older than the Shadow Fold, which was created around 400 years ago.

How old is the Darkling in human years?

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.

Did the Darkling really love Alina?

Ben Barnes himself agrees, as he relayed in a separate interview with Collider's Christina Radish in which he admits that he finds the Darkling's abuse of power disturbing, but also believes the Darkling genuinely fell for Alina: “I certainly judged the character.