Your Brain is an Organ
Remember that robust refers to the kind of thing that does not easily break under pressure, and anti-fragile refers to the kind of thing that actually gets stronger under pressure.
The heart is largely made up of cardiac muscle, which is why exercising your heart by jogging is good for your heart. This is anti-fragile. Your liver is an organ, which is why it can probably tolerate excessive amounts of alcohol, but ultimately the drinking will wear your liver down not make it stronger. This is robust.
My society has a popular saying: "the brain is a muscle," often followed by the words, "use it or lose it." The Use it/Lose it part accurately depicts the anti-fragile quality that they're trying to bestow upon the brain. But it's important to understand, the brain is not a muscle in any literal sense. It is an organ. This expression that I hear all the time is an explanation of what society believes, it is not evidence.
As an organ, I would expect proper brain maintenance to include avoiding harsh conditions (like excessive drinking is to the liver), and keeping a healthy diet. Harsh conditions might include traumatic episodes like near-death experiences or being kidnapped or beaten or persistent high-stress environments, or getting banged on the head really hard. Diet probably includes usual stuff like drinking water and eating veggies.
I would not expect brain exercise to work. I would not expect games that claim to "train your brain" to work. I would also not expect education to make anyone smarter. "Learning how to learn" is a neat line, but it's a broken strategy, like locking the key to the chest in the chest itself. How does one learn how to learn if one doesn't already know how to learn?
There is a way in which the objects of learning stack. Basic math leads to algebra. "Thinking like an economist," leads to various economic insights. Information better informs conclusions. Intellectual tools improve the quality of ideas. But it's important to understand that this is different from improved brain function. Your brain is doing what is has always been able to do, it just has more material to work with. Through these measures, one may enhance the material in the brain but not the cognitive performance of the brain itself. Education, to the extent it may be useful, may just be about enhancing the tool kit with a large stock of information and the internalization of good thinking rules, not "learning how to learn."
The brain does more than think. Emotional regulation is also an important function of the brain. I've heard one popular clinical psychologist say he recommends his depressed patients start getting better by eating breakfast. This makes a lot of sense when you see the brain as an organ. I've also seen time and time again people trying to will themselves out of bad habits and poor thinking styles. They always fail because all the will they had yesterday is all the will they have today, and no amount of momentary determination is going to change that.
My culture has spent decades of effort and funds trying to teach people how to be smarter. I suspect it cannot be done. Meanwhile, we have successfully raised IQs by many points by giving supplements to populations with iodine deficits. More of that please.
We naturally think biology is fixed but culture is mutable. The truth is probably closer to the other way around. So while thinking about the brain like an organ sounds pessimistic, it's really not in the grand scheme of things. It is only pessimistic toward some of the current strategies being employed to fix low intelligence. If you're attached to those cultural strategies because you value them in-and-of-themselves, rather than see them as a means to an end, then that might feel pessimistic. But I'd say you're caring about the wrong things.
Besides, pessimistic doesn't mean wrong. There may be absolutely no way to make people smarter. Fixing low-intelligence may be no more achievable than achieving immortality or living on Pluto. If that's the truth we have a lot to gain by realizing it and not pouring more resources into doing something that can't be done.