I find this works and is simple.
Subtract from 1970 because strtotime calculates time from 1970-01-01 (http://php.net/manual/en/function.strtotime.php)
function getAge($date) {
return intval(date('Y', time() - strtotime($date))) - 1970;
}
Results:
Current Time: 2015-10-22 10:04:23
getAge('2005-10-22') // => 10
getAge('1997-10-22 10:06:52') // one 1s before => 17
getAge('1997-10-22 10:06:50') // one 1s after => 18
getAge('1985-02-04') // => 30
getAge('1920-02-29') // => 95