Some of it is time; I don't know if I can this moment, but for the longest time I could tell you every song, in order, from Kiss' first album through probably Dynasty, and who wrote and sang it. I could do Maiden, Sabbath and AC/DC the same way. Couple other bands as well.
I'm not sure I could name all the songs in ANY order on any of those bands' most recent albums. I just don't have the time or inclination to sit with the album covers, and I'm not putting them on cassette tapes, lovingly, and listening to them over and over and over. I find myself here a LOT reading something someone wrote and thinking "Oh, I'll respond with this..." and having to lean over and pull out a CD to double-check myself. I suppose that is a subset of "gettin' old", but it's different than just the brain not working the same.