In object oriented programming, mutator method is defined to change the value of the data member(s) in the class. Since the mutator method is aimed to change the value of a class data member, it is required to take in one new value as parameter to overwrite the existing value of data member. The mutator doesn't return any output. Hence, the best declaration for a mutator method to change the age is
void setAge(int newAge);
The new value of age is passed to parameter <em>newAge</em> and the <em>void </em>means the method doesn't return any value as output.