Harry Potter was 11 years old in the first book of the Harry Potter series entitled “Harry Potter and the Philosopher’s Stone” in the UK edition and “Harry Potter and the Sorcerer’s Stone” in the US edition. In fact, it was during Harry’s 11th birthday did Hagrid show up in his aunt’s house to deliver a letter from Hogwarts informing him that he is a wizard and has been accepted into the Hogwarts School of Witchcraft and Wizardry.
Harry left the Dursley with Hagrid wherein he first saw Diagon Alley and Gringotts Wizarding Bank. It wasn’t until a month after did he meet Hermione Granger and Ron Weasley inside the Hogwarts Express located in King’s Scross railway station.