Can you really say that unless you switched fields multiple times? Of course you'll pick up on math and physics faster in high school than in college or postgrad, but that's because the problems get way, way harder as you progress. I've found that even in my late 30s I can still easily pick up new skills outside my field of expertise as long as I start with the basics that could also be picked up by a high-schooler. I started learning a new language last year and thanks to modern study apps, I actually find it easier today. Of course it will still take a long time to become an expert, but I'm not sure it would need more total hours than if I had started 20 years ago. It just gets more difficult to allocate the necessary hours for learning.

> 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.