You have all your fat cells for life by age 12. Gaining and losing will only increase or decrease what little fat cells are left. That's why you have to gain so much for the area to become as full again.
It's very hard to lose or gain fat in the face. The double chin will not come back except if there's a major weight gain. I am 5'10 and 130lbs and I have chin fat, if it could go away by losing weight it would already be gone.
