Most 12 year old boys and 12 year old girls are between 58 and 59 inches tall.
That would be around 4 feet 6 inches tall for a 12 year old boy and 12 year old girls.
Both boys and girls at 12 years of age weigh in around 90 to 93 lbs.
Of course some 12 year olds may be shorter or may be taller than another 12 year old and it depends on how tall the family is and the genes passed down in the family.