Sometimes I'm really surprised that people have such a hard time extrapolating from their own experiences learning to children and school.

Individuals will tend to learn things at their own pace. Individuals will tend to learn different things at different paces.

Example: I despised history in school. The only subject I ever scored less than a B in during a quarter. I discovered it was mostly just down to how it was being taught, as a series of factual but mostly disjointed events. I found that I enjoy learning about history, as long as I'm understanding the complex causation and linking between those different events.

Example: I've hired people that had zero experience in one of our programming languages. They typically end up completing their first PR in that language within a month or two -- with assistance, of course. People tend to pick things up pretty fast when it's part of their job expectations. (I always make it clear during interviewing and the hiring process that such an expectation would be present.)

Example: My son didn't begin to talk until after his third birthday. Now he is almost entirely caught up, with only minor speech issues.