public static int getAge(Date birthDay) throws Exception {
        Calendar cal = Calendar.getInstance();

        if (cal.before(birthDay)) {
            throw new IllegalArgumentException(
                "The birthDay is before Now.It's unbelievable!");
        }
        int yearNow = cal.get(Calendar.YEAR);
        int monthNow = cal.get(Calendar.MONTH);
        int dayOfMonthNow = cal.get(Calendar.DAY_OF_MONTH);
        cal.setTime(birthDay);

        int yearBirth = cal.get(Calendar.YEAR);
        int monthBirth = cal.get(Calendar.MONTH);
        int dayOfMonthBirth = cal.get(Calendar.DAY_OF_MONTH);

        int age = yearNow - yearBirth;

        if (monthNow <= monthBirth) {
            if (monthNow == monthBirth) {
                if (dayOfMonthNow < dayOfMonthBirth) age--;
            }else{
                age--;
            }
        }
        System.out.println("age:"+age);
        return age;
    }

------解决方案--------------------

那就要年龄具体到月才行，比如满20周岁多3个月5天 输入 20，3，5 作为参数，这样可以推算，否则只知道周岁，只能推算出生年

for example
Java code
public static String getBirthday(int ageYear, int ageMonth, int ageDay) {
    Calendar c = Calendar.getInstance();
    c.add(Calendar.YEAR, -ageYear);
    c.add(Calendar.MONTH, -ageMonth);
    c.add(Calendar.DAY_OF_MONTH, -ageDay);
    return String.format("%tF", c.getTime());
}

//调用
String birthday = getBirthday(20, 3, 5);
System.out.println(birthday);