momentjs:

/* The difference, in years, between NOW and 2012-05-07 */
moment().diff(moment('20120507', 'YYYYMMDD'), 'years')

function change(){
    setTimeout(function(){
        var dateObj  =      new Date();
                    var month    =      dateObj.getUTCMonth() + 1; //months from 1-12
                    var day      =      dateObj.getUTCDate();
                    var year     =      dateObj.getUTCFullYear();  
                    var newdate  =      year + "/" + month + "/" + day;
                    var entered_birthdate        =   document.getElementById('birth_dates').value;
                    var birthdate                =   new Date(entered_birthdate);
                    var birth_year               =   birthdate.getUTCFullYear();
                    var birth_month              =   birthdate.getUTCMonth() + 1;
                    var birth_date               =   birthdate.getUTCDate();
                    var age_year                =    (year-birth_year);
                    var age_month               =    (month-birth_month);
                    var age_date                =    ((day-birth_date) < 0)?(31+(day-birth_date)):(day-birth_date);
                    var test                    =    (birth_year>year)?true:((age_year===0)?((month<birth_month)?true:((month===birth_month)?(day < birth_date):false)):false) ;
                   if (test === true || (document.getElementById("birth_dates").value=== "")){
                        document.getElementById("ages").innerHTML = "";
                    }                    else{
                        var age                     =    (age_year > 1)?age_year:(   ((age_year=== 1 )&&(age_month >= 0))?age_year:((age_month < 0)?(age_month+12):((age_month > 1)?age_month:      (  ((age_month===1) && (day>birth_date) ) ? age_month:age_date)          )    )); 
                        var ages                    =    ((age===age_date)&&(age!==age_month)&&(age!==age_year))?(age_date+"days"):((((age===age_month+12)||(age===age_month)&&(age!==age_year))?(age+"months"):age_year+"years"));
                        document.getElementById("ages").innerHTML = ages;
                  }
                }, 30);

};

我会选择可读性:

function _calculateAge(birthday) { // birthday is a date
    var ageDifMs = Date.now() - birthday.getTime();
    var ageDate = new Date(ageDifMs); // miliseconds from epoch
    return Math.abs(ageDate.getUTCFullYear() - 1970);
}

免责声明:这也有精度问题，所以这也不能完全信任。它可以关闭几个小时，几年，或在夏令时(取决于时区)。

相反，如果精度非常重要，我建议使用一个库。还有@Naveens的帖子，可能是最准确的，因为它不依赖于一天中的时间。

短而准确(但不是超级可读):

let age = (bdate, now = new Date(), then = new Date(bdate)) => now.getFullYear() - then.getFullYear() - (now < new Date(now.getFullYear(), then.getMonth(), then.getDate()));

我对之前的答案做了一些更新。

var calculateAge = function(dob) {
    var days = function(date) {
            return 31*date.getMonth() + date.getDate();
        },
        d = new Date(dob*1000),
        now = new Date();

    return now.getFullYear() - d.getFullYear() - ( measureDays(now) < measureDays(d));
}

我希望这对你有所帮助

我认为可以简单地像这样:

function age(dateString){
    let birth = new Date(dateString);
    let now = new Date();
    let beforeBirth = ((() => {birth.setDate(now.getDate());birth.setMonth(now.getMonth()); return birth.getTime()})() < birth.getTime()) ? 0 : 1;
    return now.getFullYear() - birth.getFullYear() - beforeBirth;
}

age('09/20/1981');
//35

也适用于时间戳

age(403501000000)
//34