The average 17 year old weighs around 120 lbs and is around 64 inches in height.
However some 17 year olds may weigh a bit more than 120 lbs and some may weigh a bit less.
I weighed 118 lbs when I was 17 years old and the doctor said I was at a healthy weight.
When my younger sister turned 17 she weighed 125 lbs which was normal for her.
So not everyone weighs the same weight at the same age and that is normal.
However if you're under 118 lbs and 17 years old then that may be a bit underweight but it can vary from person to person.
A 17 year old usually weighs at least 120 lbs but there's no definite answer as all people weigh different amounts as it also depends on how they are shaped and built.