https://secure-media.collegeboard.org/digitalServices/pdf/research/2019/Program-Summary-Report-2019.pdf lists the most common AP tests taken in 9th grade as:
9th grd Total AP exam
151548 314825 Human Geography
26704 313317 World History
10813 96105 Computer Science Principles
9789 187133 Spanish Language & Culture
7560 172456 Environmental Science
5546 260816 Biology
Except for Human Geography (which appears to be used as an honors social studies option for 9th grade students in many schools), taking AP tests (and presumably the associated AP courses) in 9th grade is uncommon compared to the total volume of AP tests.