How Old is Too Old to Play RPGs?
I've been playing over 40 years, and I still like a bit of detail in my mechanics. Truthfully, D&D5 is a bit fuzzier on rules than I'd like, and I miss Rolemaster (yes, I realize there is a version still around, but no one really likes that much detail these days).
About the best detail oriented game I can find is Gurps, which is also an old favorite, but it, too, seems to be falling out of favor, juts more slowly.
I don't think it's age so much as a general trend in game rules being published. More are coming out with less detailed mech than used to be usual, and folks gets used to what is available to play, coloring their preferences by resetting what they are familiar with.