I think modern AI or for that matter engineering is fairly broken model for how the human brain or the human body works; and lead to these silly conclusions: It

I think modern AI or for that matter engineering is fairly broken model for how the human brain or the human body works; and lead to these silly conclusions: It is easy to make a machine that adds two numbers therefor adding numbers cannot be "cognitive hard". Likewise it is easy to make a machine that runs fast, lifts a heavy object or stands one leg therefore ... As for the most challenging activity for keeping sharp at old age: My guess is directly interacting with other people. But if you think lifting weights work for you, then I am not going to argue against you.