In a study conducted at the National Institute of Mental Health, researchers followed the development of the cortex, the thinking part of the brain, in 307 children. Using repeated magnetic resonance imaging scans from childhood to the latter teens, they measured the thickness of the cortex of children of age t years with the highest iq -121 to 149. These data lead to the model
S(t) = 0.000989t^3 -0.0486t^2+0.7116t+1.46 (5 < t< 19)
At what age would the cortex of children with superior intelligence reach maximum thickness.