hehe, i know exactly what you mean, i once tried to audition for fred in "once upon a mattress" when i was thirteen (needless to say, i didn't get it!
) but i just tried to make myself look older, i wore more makeup (but not too much, that just makes you look younger! ha) and a simple-yet-makes-you-stand-out kind of outfit (y'know, that standard audition outfit! haha) and just generally try to act mature. when i was in this show a few years ago, our nancy was around 30 but she was also "in" with the director (ahh, gotta love theater politics!) but the girl who played nancy's friend, bet, was only about thirteen but was chosen because she looked much older and had a fantastic voice, so i think a seventeen-year-old would be possible, especially if there aren't too many other people auditioning for the role. oh and i don't think it would be a good idea not to write your age... it might seem immature, you know?
well good luck! i hope you get the part, everyone deserves the opportunity to play their "dream role" at least once in a lifetime!