First I'll address the point about cognitive testing found in the article. "So to reassure myself that nothing was amiss, just before tackling French I took a cognitive assessment called CNS Vital Signs, recommended by a psychologist friend. The results were anything but reassuring: I scored below average for my age group in nearly all of the categories, notably landing in the bottom 10th percentile on the composite memory test and in the lowest 5 percent on the visual memory test." And later the author writes, "After a year of struggling with the language, I retook the cognitive assessment, and the results shocked me. My scores had skyrocketed, placing me above average in seven of 10 categories, and average in the other three. My verbal memory score leapt from the bottom half to the 88th — the 88th! — percentile and my visual memory test shot from the bottom 5th percentile to the 50th. Studying a language had been like drinking from a mental fountain of youth." An alternative explanation is that the author enjoyed a practice effect from taking the test more than once, and didn't necessarily gain much from his effort to learn French. To tease out which explanation makes more sense, we would have to know more about score stability ("reliability" in the psychometric sense) of the CNS Vital Signs test, and what validation studies have show about practice effects from repeated test-taking on that test.
I grew up as a monolingual native speaker of General American English in the heartland of that dialect in the upper Midwest of the United States. My first foreign language instruction in school occurred in the fourth grade of elementary school. My school district was very unusual among American school districts of that era in having mandatory German classes each week in fourth, fifth, and sixth grade, promoted and developed by a school district employee who was a refugee from Germany. So everyone I went to elementary school with had early exposure to foreign language learning--at least early compared to most other American school pupils.
I studied more German in junior high, moving to another school district in ninth grade that didn't offer German at all in junior high, even though that state (Wisconsin) has a higher percentage of German-descended persons than any other state in the United States. In tenth grade I moved back to my home town in Minnesota, and took one more year of high school German, which I bombed at after the time off occasioned by my move out of state. The following year in eleventh grade I started taking Russian, which went more smoothly for me, in part because German grammar got me ready for Russian grammar, and in part because the teacher was more kindly and more engaging and I had classmates in Russian class who were highly motivated to learn languages and shared their love of languages with me.
I went into university as an intending Russian major, hoping to take Chinese on the side as an elective course. But as soon as I started taking Chinese classes, I fell in love with that language--no declensions of nouns, no conjugation of verbs, and mostly indication of grammar by word order, as in English--and eventually switched my major course to Chinese when I had to formally declare a major and dropped taking Russian classes after a while. Chinese is of course especially challenging when it comes to achieving full literacy. (Chinese is plenty challenging for native speakers of the spoken language to achieve full literacy in the Chinese Han character writing system.) But I persisted, and enjoyed along the way learning about methods of language study from the writings of John DeFrancis, the dean of teachers of Chinese language to a whole generation of American students.
As I left the United States for a first stay overseas, with a university degree in Chinese language, I could read a book in Modern Standard Chinese reasonably comfortably and could also read some of the classic texts in Literary Chinese, but I could barely keep up with a live Chinese conversation. Landing in Taiwan in 1982, I had a headache soon after I arrived, and it was immensely gratifying to be able to go to the corner drugstore and say, "I would like to buy aspirin" and be understood well enough to get what I wanted. That relieved my headache even more than taking the medicine. Before I had left, I had spoken to two different Americans who had each spent a year living in Taiwan after two years of studying Chinese in the United States. They gave me what seemed like very contradictory descriptions of how they were able to get along linguistically in Taiwan with that much background in Chinese. One said, "It was great. As soon as I got of the plane I found out I could use my Chinese to talk to people." The other said, "It was terrible. For the first six months I was there I could hardly understand a thing that people were saying around me." I found out that they were BOTH correct, as I could communicate my needs, when I had them, especially around local people who were used to meeting American students, but I had the worst time for the longest time really understanding radio broadcasting or live conversation. I was very gratified when I could first listen through the full weather forecast on the radio and understand every word and phrase--months after arrival.
I have studied other languages, both "dead" classical languages and living languages from various language families. Every language on earth has purely arbitrary features that make it differ from other languages, and what makes a language "easy" or hard is mostly resemblance to a language a learner already knows. (German felt very easy indeed to me after I had studied Russian and classical Greek, for example, even though it had seemed hard earlier.) Young people in Taiwan used to tell me that Japanese is "easy" in about the same way that many Americans say Spanish is easy. There are many opportunities to learn each language in each of those places, but to learn them well is not particularly easy, really.
My experiences as a language learner have turned into advice I share on the World Wide Web[1] and here on Hacker News.[2] Fortunately, there are a lot of tips (one could say hacks) that help learners learn foreign languages better. I admire the many participants on Hacker News who participate while using English as a second language. That is not easy. Few Americans could participate in online discussion in any language other than English. I'm not sure I think that studying Chinese and other languages I've studied really strengthened my cognition, but I am glad for the experiences I had overseas and think that even Americans who will never leave the United States should make the effort to learn at least one foreign human language.
[1] http://learninfreedom.org/languagebooks.html
[2] https://news.ycombinator.com/item?id=6302276#up_6302816