Most people think that when they finish school, then no more study is required. On the same principle if an athlete develops a strong body and then stops training, their muscles grow weak and waste away. Likewise, the mind grows weak, if we stop exercising it.
Make it a rule in your life to study for 30 minutes a day, whether that is reading about something in a book, online or watching something in nature. Even practice adding and subtracting in your mind for a few minutes; so instead of reaching for the calculator, try to do without.
Many people do not think at all, they think that they think.
Memorizing anything strengthens the memory. Try to memorize a poem, quotation or even a shopping list, anything to improve your brain health.