What should my bmi be at the age of 13 with a height of 54' and the weight of 6 stone 3lbs. my bmi is 14 and i am wondering how underweight this is?

Your BMI would be 14.9, which is underweight. Anything less than 18 is considered underweight. To be in the "ideal" weight range for a height of 5'4'' or 164 cm would be about 120-130 lbs, or 54-59kg, or about 9 to 9.5 stone.