Languages in general.
Maybe for first languages, but it's hard to do that experiment. It's widely held false belief that children are good at learning second languages. wikipedia has the hilarious quote:
Certainly, older learners of a second language rarely achieve the native-like fluency that younger learners display, despite often progressing faster than children in the initial stages. This is generally accepted as evidence supporting the [critical period hypothesis].
NPR reports on a study giving volprioc acid to adults and training them on pitch (singing):
Brain plasticity is useful for a whole lot more than learning pitch. As the article notes it would be invaluable for training one's ear to pick up sounds of foreign languages, but also it seems reasonable to this commentator that high levels of plasticity during rationality training or other forms of self-development would result in more transformative results.