Javascript:
This probably does more than you are asking, but will give you the age in years, months, and days.
Like this:  8 years, 7 months, 21 days old
This works well if you have a site for a newborn only a few days old.
This is something I used on my daughters website almost 9 years ago 
    function GetMyAge()
{
<!--
birthTime = new Date("July 25, 2004 00:00:00 GMT-0600")
todaysTime = new Date();
<!-- Parse out specific date values
todaysYear = todaysTime.getFullYear()
todaysMonth = todaysTime.getMonth()
todaysDate = todaysTime.getDate()
todaysHour = todaysTime.getHours()
todaysMinute = todaysTime.getMinutes()
todaysSecond = todaysTime.getSeconds()
birthYear = birthTime.getFullYear()
birthMonth = birthTime.getMonth()
birthDate = birthTime.getDate()
birthHour = birthTime.getHours()
birthMinute = birthTime.getMinutes()
birthSecond = birthTime.getSeconds()
<!-- Adjusts for Leap Year Info
if ((todaysYear / 4) == (Math.round(todaysYear / 4))) {
   countLeap = 29}
else {
     countLeap = 28}
<!-- Calculate the days in the month
if (todaysMonth == 2) {
   countMonth = countLeap}
else {
     if (todaysMonth == 4) {
        countMonth = 30}
     else {
        if (todaysMonth == 6) {
           countMonth = 30}
        else {
           if (todaysMonth == 9) {
              countMonth = 30}
           else {
              if (todaysMonth == 11) {
                 countMonth = 30}
              else {
                 countMonth = 31}}}}}
<!-- Doing the subtactions
if (todaysMinute > birthMinute) {
   diffMinute = todaysMinute - birthMinute
   calcHour = 0}
else {
   diffMinute = todaysMinute + 60 - birthMinute
   calcHour = -1}
if (todaysHour > birthHour) {
   diffHour = todaysHour - birthHour + calcHour
   calcDate = 0}
else {
   diffHour = todaysHour + 24 - birthHour  + calcHour
   calcDate = -1}
if (todaysDate > birthDate) {
   diffDate = todaysDate - birthDate + calcDate
   calcMonth = 0}
else {
   diffDate = todaysDate + countMonth - birthDate  + calcDate
   calcMonth = -1}
if (todaysMonth > birthMonth) {
   diffMonth = todaysMonth - birthMonth + calcMonth
   calcYear = 0}
else {
   diffMonth = todaysMonth + 12 - birthMonth + calcMonth
   calcYear = -1}
diffYear = todaysYear - birthYear + calcYear
<!-- Making sure it all adds up correctly
if (diffMinute == 60) {
   diffMinute = 0
   diffHour = diffHour + 1}
if (diffHour == 24) {
   diffHour = 0
   diffDate = diffDate + 1}
if (diffDate == countMonth) {
   diffDate = 0
   diffMonth = diffMonth + 1}
if (diffMonth == 12) {
   diffMonth = 0
   diffYear = diffYear + 1}
if (diffYear != 1)
    YearPlural = "s"
else
    YearPlural=""
if (diffMonth != 1)    
    MonthPlural = "s"
else
    MonthPlural=""
if (diffDate != 1)     
    DatePlural = "s"
else
    DatePlural=""
if (diffYear == 0 && diffMonth == 0)
    return (diffDate + ' day' + DatePlural + ' old.  ');
else if (diffYear == 0)
    return (diffMonth + ' month' + MonthPlural + ', ' + diffDate + ' day' + DatePlural + ' old.  ');
else
    return (diffYear + ' year' + YearPlural + ', ' + diffMonth + ' month' + MonthPlural + ', ' + diffDate + ' day' + DatePlural + ' old.  ');
}
// -->