Calculate Age in Gravity FForms

I'm trying to calculate the age from the date of birth provided by user in Gravity Form. Now the thing is that I want the age to be stored in a hidden field in the gravity form so that I can use conditional login on that hidden field.

I've added a datepicker for the date of birth, but not sure if my function is ok to calculate the exact age and return it to a parameter which I can use in a hidden field.

Here is the code I've written:

add_filter("gform_field_value_age", "_populate_age");
function _populate_age( $result, $value, $form, $field ){
    $age = date('Y') - substr($value, 6, 4);
    return $age;

}

I know my function is wrong but I was hoping someone can help me out on this.

via Chebli Mohamed