I have developed B&W 35 mm film that was 21 years old. I had no idea what was on them, and it was a thrill to view them for the first time. The first roll came out very washed out.
On the next rolls, I left them in the developer for twice as long as normal, and they came out OK. I just used contrast filters when I printed them to get them back the way they should have been.
The hard part was rolling them on to the reels in the dark, the film was very hard to straighten out and tried to roll back up tight.