void calculateAgeDifference() {

int ageDifferenceInYears = this.myAge() - this.theirAge();

return ageDifferenceInYears ;

}

or better

void calculateAgeDifference() {

return this.myAge() - this.theirAge();

}

or even better

void calculateAgeDifference() {

timeDuration ageDifference = this.myAge() - this.theirAge();

return ageDifference ;

}