# Wald test on marginal effect

Say I have the following regression equation:

$$Wage_i = YearsOfEduc_i + Age_i + NumOfSiblings_i + u_i$$

How would I go about peforming a wald test of the hypothesis that for an individual with median Years Of Education, that the marginal effect of age is not affected by the the number of siblings the indiviual has?

How would I form a null hypothesis?