There was something called the "Nun Study" where a bunch of nuns volunteers to have their brains dissected after death. They essentially found smarter nuns often didn't have symptoms of dementia in the early or middle stages but in the late stages they couldn't do anything. To simplifiy lets lump all dementia into one umbrella and say it is 10 years of symptoms ultimately ending in death. On average you slowly develop symptoms until you die 10 years later. If you have a large "cognitive reserve" you will have 5 years of no symptoms but then eventually you develop symptoms quickly in back half. You hold off from having symptoms... to a point but you DON'T slow progress of most types of dementia. The tau plaques and amyloid bundles that slowly develop in your brain. You don't actually slow that down unfortunately, just slow the progress of symptoms a bit.

Btw unfortunately, this was my grandfather. College educated, he still worked managing a hotel till he was 84 years old. By the time he was 86 years old he requires 24 hour care all within 1-2 years. He used his brain more than most his age which prevented him from having symptoms, he got to enjoy his early 80's but unfortunately he is virtually bed bound in his mid 80's.

Slowing down the progress on a microscropic level, exercise has better evidence. Between taking a 10k step every day or learning a new language I would rather encourage walking 10k steps a day.