You've been declaring types inside the element declarations:

<xs:element name="person">
<xs:complexType>
  <xs:sequence>
     <xs:element name="name" type="xs:string"/>
     <xs:element name="age" type="xs:integer"/>
  </xs:sequence>
</xs:complexType>
</xs:element>

But you can name a complex at the top-level types:

<xs:complexType name="Person">
  <xs:sequence>
     <xs:element name="name" type="xs:string"/>
     <xs:element name="age" type="xs:integer"/>
  </xs:sequence>
</xs:complexType>

And then you point to them with elements:

<xs:element name="person" type="my:Person"/>
<xs:element name="student" type="my:Person"/>
<xs:element name="parent" type="my:Person"/>

Keep in mind that types can't exist in a document without an element declaration.