Actually, I think every GT level math kid by 6th grade should try AMC type problems although not necessarily in the stressful rush of a competition setting. DD, who is not mathy, has been doing the AMC 8 competition since 4th grade. She scores above average, just not enough for any award levels, but that is not the point. I push her to do it for the reasons you stated and because I find the level of math practice in her classrooms insufficient. While there are plenty of word problems and the occasional non-routine problem in her curriculum, I don't think that she is getting enough exposure to develop real problem-solving skills.
