I remember hearing you weren't supposed to place magazines that were too old either. If you're getting rid of old mags regularly, there's no reason why you'd have 20-year old magazines.
Unless it's somebody who wants to catch the eye of JWs who are leaving them there. And "Why are you looking forward to 1975?" would be a good eye-catcher. Probably an old-timer who no longer believes and has old magazines from years and years ago.
Or, some well-meaning JW inherited them and decided they'll do some good work by being placed. And let's face it, sure these things contain "old light", but some of that is so esoteric. The average person reading the occassional JW magazine isn't going to pick up on those bits. If anything, the public will read the fluff stuff anyway, not the study articles.