Вычисление возраста с помощью Zend_Date 

< ?php

class Fr_Date extends Zend_Date
{
    public function getCountYear($date)
    {
        $date = new Fr_Date($date);

        $this->subYear($date->get(Zend_Date::YEAR))
             ->subMonth((int)$date->get(Zend_Date::MONTH) - 1)
             ->subDay((int)$date->get(Zend_Date::DAY) - 1);
 
        return $this->get(Zend_Date::YEAR);
    }
}