which is a correct nonagist sentence abd why?

Question

Dr. Phil counseled the mother of a four-year-old who weighs 150 pounds.
5b. Dr. Phil counseled the mother of a child who weighs 150 pounds.

Answer 1

I suppose the correct answer is supposed to be B.

But it's ridiculous in this context. A weight of 150 pounds is normal for 15-year-old, 6-foot-tall boy. Using the child's age is necessary in the context of this sentence.