As I said in my first post, different countries could certainly have different polacies. In Tim's story, Für Das Größere Wohl, Muggle-borns receive their letter exactly on the their eleventh birthdays (Godric help the student who was born on August 31st.).

And in my own stories, students in the United States get their letters two years in advanced, while students in Australia get their letter a year before they would be set to go to school.