Taken literally, your declaration says the set of valid Age elements includes <Age><18>45</18></Age>
and <Age><25+>33</25+></Age>
, though not <Age><45>18</45></Age>
.
But '18' and '25+' are not allowed by XML as names of element types; like names of variables in most programming languages, names of element types in XML must begin with a letter. (They are also not allowed to contain blanks, so the element names "Full Size" and "Standard Elite SUV" will also raise errors.) So the examples given in the preceding paragraph are not XML, and you cannot define an XSD schema against which they would be valid.
It's not clear whether you need to review the basics of XML or the basics of XSD, but your work developing a schema will be more pleasant and productive if you have a better grasp of the technologies you are trying to use; that helps you work with the grain of the technology and not against it.