Well, smart kids do tend to stay smart: 117 is still above average. It's just that they don't necessarily stay as unusually smart, for the same reason that unusually dumb kids don't stay exactly as dumb (regression to the mean cuts both ways). When you filter for kids with 130IQs at age 5, you're picking out kids who are smart, but also ones who were in a great mood that day, who biologically developed earlier than their peers, who got lucky in avoiding infections or injuries or the many other idiosyncratic things that damage intelligence, and so on. (And the opposite for those who score very unusually low.) That luck is not guaranteed to repeat itself, so... they regress to the mean. As they get older, more of the brain is developed and has reached its limits, and there's less noise from things like whether one happened to develop working memory a bit faster than one's peers as people hit their genetic and physiological limits, and the predictive power of an IQ test score increases considerably. By middle school, you're talking more like r=0.9.