Hi, eric256.
My wife teaches LOGO to her 7th graders. (she can't start any younger 'cause 7th grade is the first time she has them) Our school is also introducing some of the elements of algebra in the lower grades as other posters have said. The earlier that starts, the more time the children have to absorb and wrap their minds around the concepts. The statement "Before 7th and 8th grade, children have a lot of trouble with abstract thinking" was a generalization and no disrespect was intended to any age kids, some of whom are very capable. (Perhaps "they" will find that abstract thinking ability is something that can be nurtured and developed in much younger kids, if some effort is put into it.)

The test my wife gives is intended to find those kids who are ready for the first year of high school algebra in the 8th grade. The ones who pass her "pre-algebra" test usually pass the course and do well in algebra 2 in high school.