I'm 5' 8.75" tall and a 19 years old female. i have an athletic build, used to be very muscular but less now. how much should i weigh? avg and thin

There is a range: YEARS ago, when I was in medical school, we were taught that ideal weight was 100# for a woman 5 feet in height; then add 5 pounds per inch, so for 5'9"--145 pounds. This is pretty close. If more muscular, a little higher. So I would say a range of 140 to 155 pounds would be ideal. You could also aim for 22-26% body fat. Good wishes:)