> For example, an average observation, my father on his late 70s beat people in their 20s in doing mental math not because he is a genius because his generation had a better general education in math than the people at 20s.

You don't need a 'better general education in math' to be good at mental arithmetic. In fact general education in math' is pretty much irrelevant to mental arithmetic.

To be better at mental arithmetic, you need to learn and practice mental arithmetic.

I am willing to bet your father did exactly that when he was younger, compared to today's 20 year olds?

Your father might or might not also have had a great general math education. But that's pretty much irrelevant for the topic at hand.

(I'm a mathematician by training, so I have a pretty good 'general education in math'. But how good my mental arithmetic is depends very heavily on whether I've recently had reason to practice.

I was my best at mental arithmetic in about 7th and 8th grade. 7th grade is when calculators are first allowed in schools where and when I grew up. If, like me, you keep forgetting your calculator at home, that means all of a sudden your mental arithmetic gets a lot more of a workout than before.)