Be specific but you shouldn't narrow it down to a particular age range, because not everyone in that group will be or think the same. If anything, say something like 'mature, literate' readers or whatever, or if your work is on a particular subject/historical period then say an audience with those specific interests. If your work is more experimental/pomo, I've read a few where people have said something broad like an audience with an open mind to ____technique. You don't have to spend too much space discussing your audience, a few well-worded sentences can often be enough.
Have a quick look in the Showcases and even past high-marked RS's on here, it'll really help to see how specific people are.
And for goodness sake's, don't write 'HSC markers' for your audience. I shouldn't even have to explain why.