Read a lot (books – even novels, TIME, financial magazines). Watch documentary TV shows and news programs which tell straight news without slanting it (good luck on that btw) and do hard crosswords which require you to research the answers.
I’m 70 and I’m always eager to learn every day. I find myself reading articles about things which have nothing to do with me just because they catch my attention and are informative.
When I was younger I retained everything I read; that is no longer the case. I’m lucky now if I can remember 3 items I need at the grocery store without a list.