It is not that you get smarter, you don't get more stupid. I think it has something to do with extraordinary energy demand of the brain. 50 year olds can restore their cognitive ability to the level of 30 year olds through physical exercise. As Jordan Peterson puts it in one of his interviews: "Your brain is a very demanding organ, and if your cardio-vascular system is compromised then you get stupid... Brain is very metabolically demanding and if you are not in good physical shape then one of the things that suffers most greatly is your cognitive function".
[1] https://www.youtube.com/watch?v=29qqv-REAZk