Harry Potter is 17-years-old in the epilogue of the Harry Potter books. This is an important moment in the series, as it indicates that he is the same age as when his story began–at the start of Philosopher's Stone. This is a defining moment in the series, due to the fact that Harry had grown up so much between those two points, and displays how much he had accomplished in his lifetime.
It is important to mention, however, that the events of Harry Potter and the Deathly Hallows take place over the course of two and a half years, but the Epilogue only covers the end of the seventh year, which is why Harry is still 17-years-old.
It is worth noting that there is a discrepancy in Harry's age in the films, as the epilogue is set 19 years after the Battle of Hogwarts, whereas in the films it is set 18 years afterwards. Despite this, in both formats, Harry is the same age at the end of the series as he was at the beginning.
