Typically we prefer the patient has stopped growing to produce optimal results with coolsculpting. As most teenagers are still growing up to a certain age, unwanted pockets of fat can change in size and therefore alter one's appearance.
There is no firm age limit for coolsculpting. However, a patient must be a candidate and be able to provide informed consent. At 18 you may be able to be a candidate and provide informed consent. Patients who are younger should have parental consent, but still may not be a candidate as their body has not fully developed.