I am 13 yo and weight 13.5 stone and am 5ft 8inch tall.
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Overweight?
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Its less about your weight and more about the amount of fat you have on your body. Hell that 13.5 stone could be pure muscle for all I know :P. I remember at your age I used to gain and loose weight very fast. Just keep yourself active by doing cycling and over fun things for kids your age staying off junk food and your weight should take take of itself.