Is it healthy to be 12 years old be 4'11 and weigh 70 pounds?

That is a VERY LOW WEIGHT for anybody, no matter how old they are. The 12 year old in question needs medical help and perhaps a psychiatric evaluation immediately. I was 5'5 and 76 pounds when I was 13-14...I was also anorexic and had to be hospitalized. At that weight, your body is in starvation mode, and you run the risk of heart complications. There is something seriously wrong there...she needs help right away.