Answer
In section 4.3, on page 224 and 225, it is said that if the residuals plot (residuals vs. explanatory variable) shows a discernible pattern, then a linear model is not appropriate. Then, it is said that the spread of the residuals must not increase or decrease with the increase of the explanatory variable.
The given residuals plot (Residuals vs. Age) shows no discernible pattern and the spread of the residuals does not increase or decrease with the increase of the age. But, they are just requirements for a linear model to be appropriate.
Work Step by Step
See the picture.
