Based on past conversations with my mother (teacher) and a friend (linguistic researcher, can't for the life of me remember what it's actually called) I would actually expect that to be the slowest period of learning in this area - slowest, but not that flat. So my suspicion is that it's 80% down to lack of data.

(But equally possible that I'm proven wrong, as it certainly isn't my area of expertise and this knowledge comes from purely random conversations in the past.)

I'd like to see per gender breakdown. It's interesting to check past research[1] against new data.

[1] http://www.scientificamerican.com/article.cfm?id=are-women-r... [Girl Talk: Are Women Really Better at Language?]

Yes, the data before 15 is still pretty spotty, not too many preteens participating -- I would be very surprised if it persisted.

Thinking back to that age, school and learning was not really much of a focus for me or my peers, we were more interested in what to do with the hormones flowing through our systems.

For myself at that age vocabulary drills present in the earlier stage of schooling were subsequently dropped moving to the next level (primary school to secondary school). I presume that junior high and middle school in other systems occupy that space and might be breaking with the teaching patterns, and teacher training, of the elementary school level. But, as always, more data would be better than speculation.