I strongly doubt this. It’s rare and we have all sorts of credible theories about why it’s rare, but the decline of so-called fluid intelligence is mostly Flynn Effect: people are getting better at taking tests.

It is a possibility I actually agree with, because a true understanding probably requires a lot more than taking some classes. It probably needs a PHD on Cosmology or something else.

But let's say a shallow understanding is good enough...even just completing a General Relativity graduate course with good mark is good enough.

You should not have entertained that comment.

There is absolutely zero evidence that 35 is some mystical cut off for "understanding." That poster has NO clue what they are talking about. Seriously, feel free to ignore that comment.

As for practical advice for learning, you should look into learning how to learn and then spend about 1-2 year habituating to the proper way to acquire knowledge. The science says your (not just you, practically everyone) current intuitions and habits are incorrect; as evidenced by almost everyone in this post. Youtuber Justin Sung is pretty much second to none in terms of a practical program for acquiring these skills.

If you want general guidelines to follow to determine who's telling you the truth and who isn't use the following wikipedia article to guide you: https://en.wikipedia.org/wiki/Active_learning#The_principles...

Note: Simply reading that article and "understanding" what it is saying is not equivalent to having a study program that implements these things, and having a program that implements these things is not the same thing as actually executing on and habituating to said program. This process takes many months to years.

Best of luck.

Is the joke because Einstein published his findings on General Relativity at 36?

Devil's advocate: how many years did it take him to get to publishing this work?