Snappy responses you weren't hoping for that nonetheless answer the question quite well

From the New York Times

In the last few years, unable to hold a list of just four grocery items in my head, I’d begun to fret a bit over my literal state of mind. 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.

All this means that we adults have to work our brains hard to learn a second language. But that may be all the more reason to try, for my failed French quest yielded an unexpected benefit. 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.

What might explain such an improvement?

Regression toward the mean.