My boys were invited to Duke TIP in 7th grade. One took the SAT and was pretty excited about doing that with all the high school kids. He actually scored well for a 7th grader. He just took the SAT as a junior and scored very, very high---but I attribute that to the kind of student/ test taker he is, not b/c of the TIP test.
My youngest took the SAT 2 yrs. later for TIP. He was very nervous and very intimated by walking around this huge school with older kids and he had a very rough, stern test proctor in the room. He scored terribly and now is very afraid of having to take the SAT in high school. His confidence was shaken and he remembers that experience. It was not nearly as positive an experience the second time.
Take your child's personality into account before testing. I do not think there is an advantage to being exposed to a large std. test at a real young age. The biggest plus is that whether it goes well or not, you don't have to pay.
We have gotten very little benefit from Duke TIP, but we gets tons and tons of mail!! Lots and lots of expensive camps and summer learning. And our kids names have been circulated around to many other programs, probably as a result of TIP selling their lists. We have not done any of the classes offered.
My kids are making good grades, are in a varsity sport, a Foreign Language Club, and do community service. A family member, who is a college admissions director, said things like that weigh far more toward college than the honorary memberships and expensive summer camps. Don't know if that helps or not.