Inclusive Language: Nonagist
a. Dr. Phil counseled the mother of a four-year-old who weighs 150 pounds.
b. Dr. Phil counseled the mother of a child who weighs 150 pounds.
I'm not sure what your question is. An agist (in this context) could be someone who discriminates against people because of their age. (Although the actual meaning of agist has something to do with feeding livestock) Presumably, a non-agist would be someone who does not discriminate. I suspect that the word agist, in this context, is derived from the word agism which means discrimination because of age.
As to the Doctor Phil statements, there would be nothing unusual in having a 150 pound child if that child were a 16 year old boy. Therefore, the age of the child is essential to the meaning of the statement.