Yes. The brain is a machine, an incredibly complex machine, but still a machine. Treat it well, maintain it properly and it will work better. There isn't a magical supplement someone can take thats going to transform them into the next Gauss. And I'm not going to wake up and find out that switching motor oils has turned my 14 year old Saturn into a shiny new BMW either. That doesn't mean my car is going to work equally well no matter what happens to it. So yes, environmental factors can and do have a lasting effect individuals intelligence at any stage of their life. The ones that make you dumber (childhood iodine deficiency, for instance) are more obvious, but it's a two way street and they prove the principle. Your question is too broad far much more of an answer then that.