Since the 18 pages are obsolete, you should no longer care, if they receive traffic. Pointing them to unrelated content using 301 redirect will not hurt your rankings.

How the obsolete pages received the traffic in the first place should be investigated. Then, use a similar technique for the newer pages.

The way I do this is using a meta refresh tag pointing to the newer pages, but I also include a noindex, nofollow tag to avoid being penalized for this type of redirect. After a time, I elimnate the old pages and yes, I use a custom 404 error page as well.

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DrTandem's San Diego Web Page Design, drtandem.com