Emma Watson was ten years old when she was cast as Hermione Granger. She was eleven years old when started playing the character of Hermione Granger for Harry Potter and the Philosopher's Stone. She was twenty-one years old when the last movie Harry Potter and the Deathly Hallows came out. Daniel Radcliffe was eleven when he was cast as Harry Potter and Rupert Grint was twelve when he was cast as Ron Weasley. Therefore, the movie series spanned over roughly ten years and the trio of the main cast were almost teenagers when they started acting in the movies and were adults by the time they bade goodbye to the franchise.