I'm asking this because I get called it all the time. I'm male, 17 (b'day was 2 days ago), and only 5'6.
I used to be a hefty 10 stone when I was 13/14 years old, and I was almost obese I presume.
I lost weight when I was 15 by running and eating less (about 1300 calories a day). I went down to my worst of 7 stone when I was 16 and people would describe me as skeletal and my family were very horrified and said I looked disgusting, ill etc. All I could notice that's changed in my appearance is my hands are very scrawny (I used to get called "chubby fingers" so I'm somewhat proud), my feet, chest, arms and face went very thin.
Since this February I've went from 45kg to 50kg (I'm now 8 stone) but people are not noticing the difference. I don't know if want them to either, because I used to be very insecure when I was fat because people always made fun of me. I've been eating junk food (I never did when I was losing weight), but at the same time drinking milk, eating meat and vegetables and just generally more food.
So, what makes you decide if someone is too thin? How can you tell? What is it about their physical appearance that's not normal? How do you decide when someone looks healthy?