I believe a very close friend of mine is (at least moderately) underweight, but he does not fully agree. He's 19, and about 5'10 and about 61kg (134 lbs). He plays a good deal of sports, mostly soccer and running, and eats mostly right (as far as I'm aware, I've known him for a while), and he's slightly muscular. It is very unlikely that he has got any major health issues, because we both had our physical exams some months back and he was mostly fine. I would appreciate some opinions on whether he is underweight, which myself and many others believe he is.
Update: thanks for all the answers. I suppose that does help to discredit my belief, and I'm glad he's alright in terms of weight. Also I can't really estimate his body fat because I've had no past experience in doing so.