Damon and Stefan Salvatore were meant to be immortal on The Vampire Diaries, but how old did they actually live to be? Portrayed by Ian Somerhalder and Paul Wesley, respectively, the Salvatore brothers were vampires born in the fictional town of Mystic Falls, which is where they return to when The Vampire Diaries begins. And while, on the outside, they appear to be young, with Stefan even attending high school, they are in fact much older than people expect.
In keeping with traditional vampire lore, Damon and Stefan were both born human but were ultimately turned into vampires from another vampire - in their case, a vampire named Katherine Pierce, who was a doppelganger of Nina Dobrev's Elena. Both Damon and Stefan were turned into vampires in 1864; Damon voluntarily became a vampire because he was madly in love with Katherine, while Stefan was turned against his will. Regardless, they both became immortal, to an extent, and lived into the 21st century.
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When Damon was turned into a vampire, he was 25 years old, and since The Vampire Diaries seemingly takes place in real time - therefore, beginning in 2009 - that means Damon was 170 years old when the series started; he was 178 years old when the show ended. Meanwhile, Stefan was 18 years old when he turned into a vampire, and was, thus, 163 years old when the show began. At the time of his death, Stefan was 171 years old.
Despite being so old, neither Damon nor Stefan showed it because they were vampires - and so, they remained 25 and 18 years old, respectively, for their entire lives. But their immortality only went so far, evidenced by Stefan's death. They could stay young forever so long as they weren't killed by any other means, which is a common trope for vampires in fiction - particularly young adult books, movies, and TV shows.
Their relationship was tested numerous times, beginning with the battle for Katherine's affection. While the two had their "break-ups" throughout the series - particularly with Damon and Stefan going 48 years without seeing each other, only to reunite in the early 20th century for their nephew's funeral - the show ended on a happy note. The Vampire Diaries series finale included a flash-forward sequence to the afterlife where the brothers reunite and embrace. Stefan even regarded Damon with a "hello brother," a callback to their reunion in the show's pilot when they both returned to Mystic Falls for the first time in years.