Can weight gain help sagging skin if underweight?

I'm 19 years old, and my BMI is around 15.8 - the lowest it's ever been. I've noticed that over the past year or so, my skin is not as tight as it once was, and it's sort of sagging in a way. It's noticeable in the eye/forehead and cheek area. I'm wondering if this can be reversed if I gain weight and work towards a healthier BMI. Or would surgery/fillers be my only option at this point? Thank you.