Files since 1999 are all still in a folder on my local drive. I've made an effort not to lose the photos and chat logs. My iTunes database dates back to 2002, and I'm disappointed that I lost my old SoundJam MP database that I used for 2 years before that.
Unfortunately, my archives continuously come under threat from forced upgrades. Upgrading iPhoto to the Photos app will lose a lot of metadata. The latest versions of iTunes removed USB sync for contacts & calendars, which really bothers me when I want to maintain my files without the cloud.
Many old documents are now unreadable (e.g. Clarisworks). I've realised that simplicity is essential for keeping long-term records. Keeping a copy as plain text is important for preservation. Just because "there's an app for that" today doesn't mean that 10 years from now, the app will still work. This is (especially) true for companies that should know better, such as budget tracking tools.
I recently went through my old records to compile a list of every rock concert I've been to, along with the location and price when possible. With heavy use of archive.org, my iCal, and my iPhoto library, I figured out all the dates, but it was a major effort. Most of them were in the last 10 years.
My dad still has his PhD thesis on a magnetic tape. Anybody who knows how to read that onto a PDP-8 should get in touch!