Most charts you will find giving ideal weights dependent upon your age and height are for people over 18. This is because when you are 13, at the onset of puberty your body is going through a lot of changes physically. You may have growth spurts and gain and lose weight quite rapidly as you settle into your adult body. This is completely normal, and expected, and will start to slow down as you reach 16 or 17.
Therefore you should not concentrate too much on what your weight is whilst you are still a child. Put energy instead into being healthy, eating well and getting plenty of exercise. If you do this you will remain at a healthy weight.
If you are worried about your weight you should talk to your family doctor who should be able to put your mind at ease, and offer advice about how to stay fit and healthy.
