According to the U.S. Centers for Disease Control and Prevention, a girl who is 17 or 18 years old and 5 feet 3 inches tall should weigh between 110 and 140 pounds. At this age, the 50th percentile for weight is roughly 120 pounds.
WebMD's Fit Kids BMI Calculator indicates that a girl who is almost 18 years old and 5 feet 3 inches tall has a healthy weight and body mass index if she weighs between 99 and 145 pounds. If she weighs under 99 pounds at this height, she is considered underweight. If she weighs 145 to 171 pounds, the BMI Calculator considers her to be overweight, and a weight over 171 pounds puts her in the obese range.