This worked for me on my test: http://devwrangler.com/dob-test/
<?php
//date in mm/dd/yyyy format; or it can be in other formats as well
$birthDate = get_field('date_of_birth');
//explode the date to get month, day and year
$birthDate = explode("/", $birthDate);
//get age from date or birthdate
$age = (date("md", date("U", mktime(0, 0, 0, $birthDate[0], $birthDate[1], $birthDate[2]))) > date("md")
? ((date("Y") - $birthDate[2]))
: (date("Y") - $birthDate[2]));
echo "Age:" . $age;
?>