Brain anatomy correlated to ease of learning a second language

In particular, the size of the left Heschl's Gyrus (HG), a finger-shaped structure in both the right and left side of the brain which contains the auditory cortex, was highly accurate in predicting ease of second language learning.

More details of the studies as explained from Science Daily:

The three studies have identified behavioral, neurophysiologic and, with the current study, neuroanatomic factors which, when combined, can better predict second language learning success than can each single factor alone.
In a behavioral study, Wong's group found that musical training started at an early age contributed to more successful spoken foreign language learning. The study participants with musical experience also were found to be better at identifying
pitch patterns before training.
In a neurophysiologic study -- again with the same participants -- Wong's team used functional magnetic resonance imaging to observe what parts of brain were activated when participants listened to different pitch tones. They found that the more successful second language learners were those who showed activation in the auditory cortex (where HG resides).
The participants all were native American English speakers with no knowledge of tone languages. In tone languages (spoken by half the world's population), the meaning of a word can change when delivered in a different pitch tone. In Mandarin, for example, the word "mi" in a level tone means "to squint," in a rising tone means "to bewilder" and in a falling and then rising tone means "rice."

Results:

As a group - and sometimes in fewer than two or three sessions -- the nine participants predicted on the basis of left HG size to be "more successful learners" achieved an average of 97 percent accuracy in identifying the pseudo words. The "less successful" participants averaged 63 percent accuracy and sometimes required as many as 18 training sessions to correctly identify the words.

I wonder when the HG stops growing, and/or how predictive this is in children or infants?