The small number of test-takers in grades 8 and below do the best on the AMC 10, according to data from
http://amc-reg.maa.org/reports/generalreports.aspx , showing the power of self-selection:

Grade Average 10 A 2013
Location: United States

avg_score,grade,#students
77.2,06,795
78.9,07,1730
79.3,08,3332
69.5,09,23236
71.6,10,26286
68.4,unknown,1337
71.4,all,56717

The data for "06" is actually for grades 6 and below.