There was an era when nobody needed a "brain gym". People built henges and pyramids - they generally toiled and spun. They never felt that the hippocampus was starting to atrophy, or that the frontal lobe no longer flexed with youth's vigour.
But then came technology. Suddenly, there were wheelbarrows and the latest labour-saving pitchfork. You didn't have to put the same intellectual effort into things - you just sort of hefted stuff about in an ale-soaked daze. If you were very lucky, you didn't even have to do that. People did it for you, and you had your ale-soaked daze all to yourself.
It was around that time that brains started to get flabby. After a hard morning's thinking, people had to go and lie down. It is a little-known fact that, had brain gyms been around in the days of old, the afternoon nap might never have existed. Instead, it became more and more common. In Mediterranean countries, people started doing it together and blamed it on the heat.
What they really needed was a good cerebral workout. Unfortunately, the concept of exercise fell by the wayside once the ancient Greeks gave up gymnastics (probably because they had to be done with nothing on). The gymnasium only came back into fashion when it was found to be a very good way of torturing schoolchildren. (Of course, they didn't get to wear much more. Oddly enough, dressing in shorts and a vest in the middle of a north European winter is probably the only fashion that was so daft it didn't catch on anywhere, except possibly Sheffield.) Then came Lycra, and the world changed again. Suddenly, everyone was in the gym exercising all the bits they didn't need while drinking water and listening to music. After that, the arrival of the brain gym was just a matter of time. Strictly speaking, these do not have to be on street corners and have joining fees. But, by a strange coincidence, two of the recommended brain-gym activities are drinking water and playing music. This must mean that all those people on running machines seeking aerobic nirvana are getting cleverer at the same time. Any time now they'll realise that joining fees are a con.
But then something else will come along, of course. It'll be brain gym, but not as we know it.