Age is not the determining factor. It is whether or not your brain functions. Many people far younger have limited brain function.
My tip for paradox grand strategy games is to watch your way through a good Let's Play youtube series, and then be prepared to slowly read your way through all of the tooltips where needed to figure things out. For EU4 I think I watched through a 100 hour Lets Play to give me the general gist, and then I knew enough to be able to explore the rest myself (to the extent that I felt I needed to - certainly you don't need to know all the details to play and enjoy the games). Once you've given some time to a Let's Play to decent extent then getting through a game is reasonably straightforward. Age is definitely not a factor, only interest and willpower! Stellaris is quite accessible, EU4 is generally well regarded. (CK3 is coming out in august or september and looks to be quite accessible too, though we won't really know until it lands.)
