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depends on your gene makeup like most things. Basic facts though are as you age your iq becomes more correlated with the iq of your parents. If you're a guy your brain has fully developed for the most part at around 25 and at that point it feels like a slow line with an infinitesimal decline constant.
The rest of this post is just my thoughts.
You mental reasoning may noticeably dip at different ages, sometimes as young as 40 but I'd say that for a person who stays healthy, doesn't stop learning or exercising their mind and also watches out to not burn it out will stay relatively capable up until 70 and even further if they don't get any serious psychiatric problems, but of course at that point the decline has become noticeable no matter what because of year of a small constant decline. I don't imagine the average 70 year old having treated their mind that way though.
