The Vampire Diaries' Elena and Katherine lived a portion of their lives as immortal vampires, but how old were they really? Both characters were turned at a relatively young age, and the show portrayed them each at different points in their lives. It is possible, however, to determine how old old Elena and Katherine really were by the show's finale.
The Vampire Diaries centered on the life of Elena Gilbert (Nina Dobrev) as she meets a pair of vampire brothers named Stefan and Damon, all while she deals with the sudden death of her parents. The brothers protect Elena and her town of Mystic Falls from supernatural threats and villains that come her way. One such threat is Katherine Pierce (also played by Dobrev). Even before she's turned into a vampire, Elena is revealed to belong to the same line of doppelgängers as Katherine. Both are descended from an ancient vampire named Amara and live as her "shadow-self."
Katherine and Elena were turned into vampires as teenagers, but the show portrayed Katherine much further along in her immortal life than Elena. The two women lived very different lives as vampires; Katherine Pierce and Elena Gilbert had unique experiences as supernatural entities on The Vampire Diaries.
Katherine was born into a wealthy life in 1400s Bulgaria, but her life took a turn for the worse in her teenage years. Her family disowned her at age 15 after she gave birth to an illegitimate child. She was banished to England and taken in by a group, later revealed to be vampires. After she learned of the group's villainous side, she tried to commit suicide as an escape. A vampire named Rose force-fed Katherine her blood, turning her into a vampire at the young age of 17. As many of the characters in The Vampire Diaries are hundreds of years old, the show features a number of flashbacks. Katherine is seen in the 1800s as she meets Damon and Stefan, and in the 1920s as she watches over Stefan. But by the time of her death in The Vampire Diaries's season 5, as a delayed side effect of taking the cure for vampirism, Katherine is over 500 years old.
Elena's vampiric turn has some loose parallels to Katherine's turn. At age 18, Elena gets in a car crash that ends with her drowning. Damon turns Elena into a vampire to save her life. Elena actually only spent a year of her life as a vampire, as she was given the cure at age 19. Shortly after receiving the cure, however, a spell placed her into a deep slumber. She's subsequently awakened in The Vampire Diaries series finale. The spell prevented her from aging, so she's able to continue on with her young life as a human.
Elena may have spent far less time as a vampire than Katherine did, but each of their experiences is vital to The Vampire Diaries. The ways in which their time as vampires differ from one another show how the power of immortality impacts different characters on The Vampire Diaries. Even though both their vampiric turns were born out of desperation and tragedy, Elena and Katherine carried the burden of vampirism in dramatically different ways.