However, the AP courses/tests most commonly taken in 9th and 10th grade (e.g. human geography, world history) are probably the least likely ones to be given subject credit in college (though a student may earn generic elective credits if s/he attends a school that is generous with credit units for AP scores).
Obviously, unusual examples like math prodigies taking calculus BC in 9th grade are a different story. Perhaps somewhat more common would be those taking the foreign language ones early due to previous knowledge of the language (heritage speaker and/or from an elementary school program that includes extensive foreign language instruction and immersion).
The College Board itself, in https://secure-media.collegeboard.org/apc/Appropriate-Grade-Levels-for-AP-Courses.pdf , suggests that AP courses are generally to be offered to 11th and 12th grade students, with a few being suitable academically prepared 10th grade students.
https://secure-media.collegeboard.org/digitalServices/pdf/research/2016/Program-Summary-Report-2016.pdf indicates that, for 2016, the percentage of AP tests taken by students in each grade, and the percentage of AP test taking students in each grade was:
Grade % of tests % of students
9th 3.8 6.4
10th 13.3 19.5
11th 38.5 36.3
12th 42.7 35.7
Note that human geography made up the majority of AP tests taken by 9th grade students, and most human geography AP tests were taken by 9th grade students. The other AP tests where the bulk were taken below 11th grade were European history and world history, both with the majority taken by 10th grade students.