I think the ages here matter. 12yrs old is typically 6-7th grade. Similar to elementary in that the kids don't really get to select their classes (other than perhaps band vs choir and which language to study) but they are still working study skills, organization, beginning puberty, etc etc etc.
By 16 they are sophmores/juniors in high school, have probably gotten through the most awkward part of puberty, worked out how to efficiently handle studying/homework, and are taking classes they've gotten to select and are interested in.
It makes sense to me that 16yr olds are in a more stable academic place than a 12yr old, and that test scores would reflect that.
