Look at my photos!

Unless you've had an eating disorder, I wouldn't worry about it, as long as you have healthy eating habits and an active lifestyle. Lots of people are that thin at your age. You'll gain weight as you grow.

For insight, I'm 5'0, 17 years old and have been naturally thin my whole life. My normal weight is around 95 pounds. You'll gain weight normally and healthily as you get older, just be aware if you start obsessing about your weight or getting anxious about it, because that indicates that you're at risk of developing an eating disorder.