Is it your actual brain or is it having a million things to worry about? Eg. if you could do a one-month retreat away from it all, would you get the same out of it as your younger self?
Of course it's also true one's younger self had other distractions, so it can be hard to compare.
The full story is I had a severe burn out (so that means exactly that : your brain is really damaged). My family is super supportive so I don't have that many things to think about.
As a young er student, I had tons of "side projects" (like coding games) :-) So I was definitely not studying all the time. Nowadays I'm 100% focus so it means I work about 8-10 hours a day on my courses. I have very good sleeping habits, I eat well (vegetables but just too much chocolate), I walk 45 minutes 5 days per week and practice taichi about 20 minutes a day.
I tend to outperform students (about 2 standard dev above mean) in practicals but for examinations, where I have to memorize the whole course, that's another story, I perform just as the average BUT I pass half the exams so it means I'm really underperforming... The stress is a big issue but the memorization is the biggest one. It just doesn't work like when I was 20. It's super frustrating.
I have a master im compsci + 2 years phd preparation. But after 20 years, data science is totally foreign to me. It's just math math math, I don't recognize anything I know (so it's super interesting, but I'm really lost :-))
Also I chose data sciences because I was scared to see some COBOL programmers who didn't jump into the web/java/you name it track and who are now old and can't make the effort to switch). So I felt, business intelligence is moving outside the realm of application developpers (because many companies now know how to to business processes with IT) to the realm of data sciences (because that's where there is more room for improvements). I hope I was not too wrong :-)
I recognize what you mean about exams. With some distance from it I now think a lot of what is needed to study for an exam is not really important, it's just incidental stuff that you need to perform in a timed setting, eg memorizing derivations.
On the practical side it makes total sense that you're better, because young people generally have not practiced long term productivity.
Of course it's also true one's younger self had other distractions, so it can be hard to compare.