<?php
function age($date){
    $time = strtotime($date);
    if($time === false){
      return '';
    }
 
    $year_diff = '';
    $date = date('Y-m-d', $time);
    list($year,$month,$day) = explode('-',$date);
    $year_diff = date('Y') - $year;
    $month_diff = date('m') - $month;
    $day_diff = date('d') - $day;
    if ($day_diff < 0 || $month_diff < 0) $year_diff-;
 
    return $year_diff;
}
?>

http://www.apphp.com/index.php?snippet=php-calculate-age-using-birth-date

This function gets birth date as an argument and it returns the age of the person; it's very useful when building communities or social media sites.

Calculate Age Using a Birth Date

PHP