This is pretty silly. Your memory obviously gets a little worse as you age, with most of the noticeable decline coming in very late age. But the artificial measures/definitions of things like "fluid" intelligence are mostly useless. Just pulling up one of the studies cited in the article which is supposed to measure the "reasoning" aspect of "fluid" intelligence, presents a huge host issues immediately [1].
Aside from the lack of randomization, you have obvious validity problems. The interpretation of nebulous words like "reasoning" as being accurately measured by e.g. accuracy on Raven matrices (construct validity?) and younger participants having been primed by recent test-taking experience while real-world reasoning skills aren't really reflected - it's all quite specious.
Real-world decisions are value-laden and constraint-laden! "Intelligence" does not mean "maximizing abstract pattern detection". If you keep your brain active with a wide range of creative, interesting problems, you will be fine apart from neurodegenerative diseases, which have real effects.