how to set select field default value

I’m trying to create a ModX document with a form.
This form contains several fields, mostly input type=text

Some fields are select fields.

This isn’t a problem but sometimes the default values are retrieved from my order database.

With the text fields I’ve did it in the following way:
$modx->setPlaceholder(‘fieldname’,’value’);

How can I do the same thing for select fields?

My idea was to create a snippet like:

$output=’<select name="Age" id="Age">’."\n";
$output.=’<option value="-"’;
$output.=">-</option>\n";
for($i=0;$i<=100;$i++)
{
$output.="<option value=\"$i\"";
if($leeftijd==$i)
{
$output.=" selected=\"selected\"";
}
$output.=">$i jaar</option> \n";
}

$output.="</select> \n";

And call this snippet in the form like [! age? &var=`[+age+]` !]

Does anyone know how I can manage this?

Thanks in advance