After the last two entries, you might be thinking "I guess I can never use text() as a node test with typed XML again". Not so. The error message reads: 'text()' is not supported on simple typed or 'http://www.w3.org/2001/XMLSchema#anyType' elements. So what's left? Mixed content, for one thing. Mixed content consists of a mixture of text and also embedded subelements.

If we change the schema to allow mixed content (this schema also allows a particular subelement):

CREATE XML SCHEMA COLLECTION mixedage AS
'<xs:schema
xmlns:xs="http://www.w3.org/2001/XMLSchema"
targetNamespace="urn:ages"
xmlns:tns="urn:ages">
  <xs:complexType name="age" mixed="true">
    <xs:complexContent mixed="true">
      <xs:restriction base="xs:anyType">
         <xs:sequence>
           <xs:element name="dogyears" type="xs:int"/>
         </xs:sequence>
      </xs:restriction>
    </xs:complexContent>
  </xs:complexType>

<xs:element name="age" type="tns:age"/>
</xs:schema>
'

Then the text() node test works with typed XML just fine:

DECLARE @x xml(mixedage)
SET @x = '
<ag:age xmlns:ag="urn:ages">This is the age in dog years<dogyears>3</dogyears></ag:age>'
-- now it works OK
SELECT @x.query('
declare default namespace "urn:ages";
/age/text()')
GO