How old are the actors in the second Harry Potter? Harry Potter is 12, Ron 1 1, Hermione 10, Harry Potter is 13, Ron 12, Hermione 1 1. Harry Potter No.3 Middle School, Harry 15 years old, Ron 14 years old, Hermione 13 years old. This is the era described in the book. Actually, Daniel was 13 years old and Hermione was 12 years old when filming the Harry Potter's Secret Room. I suggest you check the birth dates of the three little actors and watch movies. Just calculate the specific release date.