More like the physical limitations on lifespan.

But probably memory as well. If I start specializing elsewhere I'll forget the intricate details of the thing I'm already specialized in.

Don't think it works like that.

Rather, it's that you have only so much time to dedicate to things... if you're focusing very much on one, then you're not spending time with the other -- and overtime, you'll lose that other thing. If you can manage to spend a good amount of time on two things (let's say, learning a new instrument and learning a language) everyday, consistently, then you will get both.

It does work like that. As you get older your ability to retain information wanes.

That could be physical degradation from age, not necessarily the "running out of disk space".

Why does the cause matter?

Because if it's simply because of physical degradation, that means someone can keep on learning and not worry about running out of memory.

If your memory doesn't work, what does it matter how much you have?

You can create ungrounded abstractions all you you want, but as an older person who has commiserated with other older people, the common conception that it's harder to learn and retain things in older age is a real thing

Sure, but what you're describing is the difficulty of learning as age sets in. That's very different from the original assertion- "If I start specializing elsewhere I'll forget the intricate details of the thing I'm already specialized in." - unless that person was describing themselves as older.

If you wish to exclude more than half the population from your statement be explicit about it.

Perhaps fixing degradation will prove to be a less difficult task than increasing memory, as the growth of new neurons is still a very newly-studied phenomenon compared to treatment into neurodegenerative processes, and soon medical science will benefit the learning of all demographics.

Every time I learn something new, a little of the old gets pushed out of my brain. Remember that time I took that wine making course and forgot how to drive?

- Homer Simpson

Mastering a foreign language and math is not outside of realm of any human though - there's millions of people who learn both English and even more foreign languages together with math. Or anything else.

Or are you saying that non-Americans are less knowledgable because they learned one more language?

In life you usually want to be a master of one thing, but have knowledge of many other things to apply that mastery as widely as possible. Learning 10 languages at the 101 level can let you say "Hi, I'm a master of (whatever)" to many more people.

No, you want to be a master of 2 things: A human language, and something that everyone else on the planet isn’t also a master of. Mastering two things is hard. Mastering three things is near impossible. (Although If you naturally get multiple human languages, from parents from different linguistic backgrounds, for example, that’s great. And you might have a chance of mastering both. But one of them ought to be English.)