Java를 사용하여 연령을 연 단위로 안정적으로 계산하려면 어떻게 해야 합니까?

public int getAge() {
    long ageInMillis = new Date().getTime() - getBirthDate().getTime();

    Date age = new Date(ageInMillis);

    return age.getYear();
}

public class AgeCalculator {

    public static int calculateAge(LocalDate birthDate, LocalDate currentDate) {
        if ((birthDate != null) & (currentDate != null)) {
            return Period.between(birthDate, currentDate).getYears();
        } else {
            return 0;
        }
    }
}

public class AgeCalculatorTest {

    @Test
    public void testCalculateAge_Success() {
        // setup
        LocalDate birthDate = LocalDate.of(1961, 5, 17);
        // exercise
        int actual = AgeCalculator.calculateAge(birthDate, LocalDate.of(2016, 7, 12));
        // assert
        Assert.assertEquals(55, actual);
    }
}