You are probably the smartest at around 5 to 10 years old. At least when it comes to creativity and learning. Peak performance of brain capacity is probably around 20 to 25. After that you aren't getting particular less intelligent per see but stress and lack of sleep are deterrent. The risk of stroke also increases as you get older. There are probably a peak of intelligence at around age 50 when your brain is still healthy and you have acquired experience and knowledge.