I have looked, but haven't seen anything regarding being underweight. My son, being so young, is on thin side, and according to the Air force requirements would be a few pounds shy of minimum, although hes ok for the other services. Do they give a waiver for this fairly easily? Seems to me its fairly obvious he won't be so thin for long, just a matter of body weight matching up to growth and at his age (17) its not all that uncommon. I guess what I want to know is, how much of an issue is this.