codice:
  <html>

    <head>
      <script type="text/javascript" language="JavaScript">
      <!--

        function generateCategory( __hForm )
        {
          var _bornDate  = __hForm.elements["date"].value;
          var _dateToken = _bornDate.match( /^(\d{2})\/(\d{2})\/(\d{4})$/ );

          if ( _dateToken == null ||
               _dateToken[1] < 01   || _dateToken[1] > 31 ||
               _dateToken[2] < 01   || _dateToken[2] > 12 ||   
               _dateToken[3] < 1900 || _dateToken[3] > 2005 ) 
          {
            __hForm.elements["category"].options.length = 0;
            return false;
          }

          else {
            var _curDate = new Date( );
            var _typDate = new Date( _dateToken[3], _dateToken[2]-1, _dateToken[1] );
            var _gotAge  = parseInt( (_curDate.getFullYear() + _curDate.getMonth() / 100 + _curDate.getDate() / 10000 ) -
                                     (_typDate.getFullYear() + _typDate.getMonth() / 100 + _typDate.getDate() / 10000 ) );


            if ( _gotAge >= 18 && _gotAge <= 28 ) {
              with ( __hForm.elements["category"] )
              {
                options.length = 0;
                options[options.length] = new Option( "Membro Junior", "junior", false, false );
                options[options.length] = new Option( "Membro Super", "super", false, false );
              }

              return true;
            }

            else if ( _gotAge >= 29 && _gotAge <= 40 ) {
              with ( __hForm.elements["category"] )
              {
                options.length = 0;
                options[options.length] = new Option( "Membro Senior", "senior", false, false );
                options[options.length] = new Option( "Membro Super", "super", false, false );
              }

              return true;
            }

            else {
              with ( __hForm.elements["category"] )
              {
                options.length = 0;
                options[options.length] = new Option( "Non puoi diventare membro", "invalid", false, false );
              }

              return false;
            }
          }
        }

      //-->
      </script>
    </head>

    <body>

      <form>
        Data:

        <input type="text" name="date" value="gg/mm/aaaa" maxlength="10" onkeyup="generateCategory( this.form );" />



        Categoria:

        <select name="category"></select>
      </form>

    </body>

  </html>