So here we have generic discussion number #72, where we talk how neurogenesis stops at a certain age but not really and how stuff can improve your cognition but only if you're a mouse.
Also a good place to recommend The Brain That Changes Itself[1].
1. https://www.goodreads.com/book/show/570172.The_Brain_That_Ch...
Some languages are harder than others though. I leaned English in my twenties (20-25) it was easier than learning German (at 25+). Maybe that's because there is a lot of material in English compared to German (TV shows, Movies, Music).
I also tried Japanese, it's fairly easy to pick up daily phrases but reading/writing was almost impossible for me.
Learning Italian for example could be much easier, even in middle age.