Assuming that you read out those three variables into $bMonth, $bDay, and $bYear, this will round their age down so that it will say they were 5 years old instead of 5.2356 years old.
$age = floor( ( ( time() - mktime(0, 0, 0, $bMonth, $bDay, $bYear) ) / 365 * 86400) );
Remove the floor() if you wish to make it show their precise age with all the decimal ugliness.