I set out to learn Spanish almost exactly one year ago, deciding it was ridiculous to be monolingual in a world of thousands of languages. My approach was as intensive as the author's; I completed the Duolingo skill tree, read books on grammar and slang, watched, listened to, and read Spanish media and Spanish translations of English media, attended meetups, etc.

While I'm only 23, I experienced the same "mental fountain of youth" effect the author describes. At first, it all sounded like gibberish even if reading along with what I was listening to. My mind refused to accept words and concepts that didn't map one-to-one with English. I repeatedly mixed up similar looking words.

Despite constant failure, I kept at it, and eventually it just "clicked". Spanish errors started to sound like "he hitted me". I went from barely memorizing a few words a day, to successfully internalizing hundreds. And the best part is, all these cognitive benefits have transferred to other realms. I memorize stuff on Anki now just for the hell of it because it's so easy. I am about halfway to being fluent in German after only a month of study. I really can't recommend learning a foreign language enough.