Would it be bad just to eat 700 calories a day if your only 14?

Yes, it would be very bad for anyone, especially someone under the age of 18. Most teenagers need a minimum of 1,500 calories per day (more if you're active), and if you eat too little, you could end up not only slowing your metabolism down, which makes it very easy to put on body fat, but you could also cause lots of health problems later in life.