> Can you really say that unless you switched fields multiple times?

I have ;-) far too many times! Even going back and taking undergrad math coursework that my engineering curriculum didn't have like Discrete Math or Statistics got a lot harder than calculus / differential equations was when I was younger. I felt like I got less out of each hour, and also couldn't put in as many hours - not just because I have more responsibilities, but also because my brain just gets tired after fewer hours.