My parents moved around a lot when I was in grade school, as often as three times in a school year. I attended all different types of school from inner city (ghetto) to private / parochial. I'm also one of those 'mutants' who has an incredibly vivid recollection of everything that went on from the first day of pre-school. I also have many elementary school teachers in my family.
I relucantly agree with the OP in that the top students in the later years of 4-6th grade and junior high have mostly all turned out to have stable careers in ther later life. By 'stable' I mean they have chosen their careers, and are able to provide for their lifestyles as they want. Might not be rich or practicing medicine, but are comfortable because they've chosen to be. While this group typically (but not always) includes the highest I.Q.s of the bunch, it also includes a lot of average intelligence kids who are just more disciplined and driven enough to get where they are. I'll be the first to admit that once kids start getting past pre-teen it's pretty obvious to spot the ones who will be working at Target when they're 40, or, better said, not working at Target when they're 40.
You then have that pesky sub group of high I.Q. kids who scored off the charts on tests, but were generally a bit flaky and either now have careers driving a delivery truck, or are millionaires.
I would also say there is a stronger correlation between parents who give a shit in the 6th grade and where they end up.
You must know some grade-school teachers

All the ones I know will tell you the same thing in that all their best students tend to demographically come from stable homes. Within a week of the start of school a veteran grade-school teacher can typically point out with greater than 90% accuracy which kids have the best parental influence without knowing a thing about the kids homelife.