Earlier quoted context omitted.
18 is awfully old to start learning something for the first time. Imagine saying the same thing about math, reading, or history. It's harder to pick things up when you're older, and it leaves fewer people capable of it. See also: learning human languages.
This is absolutely false. I became fluent in Japanese in my 30s. Learned guitar in my late 20s. This is so wrong. You can continue learning until you are dead.
GP is not saying you can't learn. GP is saying you're unlikely to be more skilled than those who started earlier. Also, past a certain age, there's a more limited amount of time you can dedicate to learning new stuff.