Is it a varchar column in the database? php will assume european date format if you use dashes instead of slashes I think, so you may be able to use:
<?php
$birthDate = strtotime(str_replace("/","-",$rsPROFILE->getColumnVal("dogsdob")));
$ageInSeconds = time() - $birthDate;
$age = intval($ageInSeconds/(60*60*24*365));
$months = intval((($ageInSeconds/(60*60*24)) % 365)/12);
echo(strval($age) . " years " . $months . " months");
?>

