Just my 2cents:

I think one of the reasons that more boys may be identified as gifted at an earlier age is that so many of them are sent for testing for ADHD type behavor issues. DS's 3rd grade pull out has 4 boys and 1 girl in it. According to the other parents, all of the boys were identified early due to behavior problems. Our school doesn't test for gifted until the end of 3rd grade for a 4th grade start.