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From my physiology teacher, when I was studiying medicine, he says 18 to 25. don't think he was wrong.
25-30 is overweight
>30 is "obese" (see that right here right now cover from fat boy slim)
there also may be different scales. In africa 12 to 18 could be normal when it's 25 to 30 in the us, don't know :x