Today I learned about expired film

I have a bag of film that I've been toting around with me for the past 15-20 or so years. From the boxes, the film "expired" around 2002 or thereabouts.I shot a couple of rolls of it and things looked OK. I was looking at the density of the resultant negatives, and I was a tad disappointed. I developed a new roll of Ilford Delta 400 today, and it looked quite a bit different from the old Kodak T-MAX 400 TMY that I shot. I'm not talking about minute details about how it renders the picture; I'm talking about the gross level of density on the roll.So I decided to look a bit deeper into the topic. I found an article from Popular Photography about exactly this.This article confirmed my suspicions. It seems that film starts to lose speed over time. Since I'm working with two-decade-old film, I'll have to downrate the speed by about a full stop. That makes sense given my results. So the 400-speed film is now a 200, and the 100 is now 50. Of course, I can develop it longer, but that brings up other potential issues like fogging.I have enough of the film around to conduct some real experiments to see how to use it in the present. That said, I think I can safely say that I shouldn't use it for things that I really want to preserve. But, I think I'll first just try to develop a bit longer and see what the results hold.