We all do it at some point, look at our kids and think when did they get so smart? A study by the National Institutes of Health has an answer for that... age 11 or 12.
The NIH study has been tracking the brain scans of about 500 healthy American children from their birth to age 18. And they found that from age 6 to 10, the brain improves rapidly. But by age 11 or 12, a child's brain appears to reach the same level of basic intelligence and skills as an adult. From then on it's just figuring out how to use all that he or she has learned.