...the awkwardness of sitting the test with juniors and seniors ...
As others have mentioned above, it depends on the student. I am aware of students as young as 11 years old, with 7th grade placement, being curious to experience the high-stakes testing process and subsequently successfully sitting for the test with High School Juniors and Seniors... without experiencing awkwardness. Going into it without expecting any special treatment or consideration for one's age difference may be key. A government-issued picture ID was required.