MiraDry is approved for patients 18 years of age and older. That doesn't necessarily mean that miraDry would be unsafe in teenagers younger than 18, but that it has not been studied in that age group.
miraDry is FDA approved for those above the age of 18 but is done for younger patients. There are no described untoward side effects from miraDry in the young - example - breast development, etc. The treatment is 'skin deep' and does not affect underlying structures.