根据生日计算年龄可以通过Calendar实现。最简单可以考虑get(Calendar.DAY_OF_YEAR)来简单修正年龄，但是遇到生日在闰年的2月29之后，或者今年是闰年的2月29之后可能出现计算不准，误差一天。所以还是老实判断年月日好了。

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) {
                //monthNow==monthBirth
                if (dayOfMonthNow < dayOfMonthBirth) {
                    age--;
                } else {
                    //do nothing
                }
            } else {
                //monthNow>monthBirth
                age--;
            }
        } else {
            //monthNow<monthBirth
            //donothing
        }

        return age;
    }