Serving stale pages

I have to try to explain to a customer why a page that has changed is
being served from the cache and not from the web server.

Am I correct in assuming that it has something to do with the web
server not sending the Last Modified information or the Expires
information in the http header.

I have seen some threads on this list about mailing the webmasters at
these sites to get them to change their practices.

Here are two sites info from netscape browser.

www.lightspeed.net

Location: http://www.lightspeed.net/
  File MIME Type: text/html
          Source: Currently in disk cache
Local cache file: 07/cache35B385471223B01
   Last Modified: Unknown
   Last Modified: Unknown
  Content Length: 9978
         Expires: No date given
         Charset: Unknown

Location: http://squid.nlanr.net/
  File MIME Type: text/html
          Source: Currently in disk cache
Local cache file: 01/cache35B387E11323B01
   Last Modified: Thursday, 25-Jun-98 13:38:17 Local time
   Last Modified: Thursday, 25-Jun-98 19:38:17 GMT
  Content Length: 3738
         Expires: No date given
         Charset: Unknown

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Rodney D. Holm                  rodneyh@apexxtech.com
Apexx Technology, Inc.          http://www.apexxtech.com
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