Olden describes times that happened long ago. As the composer Cole Porter once wrote, "In olden days, a glimpse of stocking was looked on as something shocking. But now, God knows. Anything goes."
You can use this old-fashioned adjective to talk about bygone days or things that happened, like in a fairytale, "once upon a time." Olden almost always describes "days," or sometimes "times." While many older people like to talk about how much better the olden days were, there are plenty of them who appreciate things like the Internet and mobile phones, who don't miss the olden days at all.