How old should I be if I will be in year 11 after this summer?

You would be a year older than your classmates, as the eldest in the year would normally be born in September 1997 (The boundary.).

I was born in August 1997 as well, and i'm going in to year 12 which means you will be a "year" a head of your classmates. However, you'll only be a few months older than most people. Most people will turn 16 during year 11