The BMI is a pretty lame screening test. There is no way to distinguish fat from muscle and age is not a factor after the mid-teen years. Clearly, a 70 year old and a 20 year old don't have the same muscle/fat ratio.
A better "test" would be waist size with an age factor. While a 70 year old might have a 34 inch waist (if female), this wouldn't necessarily mean she was fat given her stick-like limbs. A 34 inch waist on a 20 year old female would spell fatness unless she was 6 feet tall or more.
Maybe we should just go back to the old Met Life tables.