What other say
"Customer insights are like bananas.
Yes. You read that right. Customer insights share the same issue with bananas. The only difference is that, with bananas, we know the issue - with customer insights, we tend to overlook it. And this is it:
They have a limited shelf-life.
They expire. Go bad. Become brown."
— An extract of a short story shared by Service Designer Niels Corsten
My two cents
Here are a few ideas I'll try to explore in the future when it comes to making sure my research reports age well:
Add the date of publication: Just having the date (visible) of when the report was made is already a piece of pretty good information for people who read it. It can help them think: "Oh, things have changed a lot since then! Maybe this isn't so accurate anymore".
Add a suggested expiration date: In your report, you could, like for yoghurts, add a date that says something like: "The learnings might be out of date in XX".
Add a reminder to review the research on the expiration date: the expiration date isn't good only for the reader, but it can also be pretty helpful for you. In fact, you could add a reminder in your calendar or to-do-list tool to review the report on that date. And if the insights are still relevant, change the expiration date. And if the data isn't relevant anymore, archive the report.
Review the research reports: From time to time, maybe once a year, have a spring cleaning of all the research that you have. Archive the stuff that isn't relevant anymore and keep the rest. This is a good complement for the research reports where you didn't have an expiration date.