When I went to medical school, the belief was that you got your full quota of brain cells at birth. It was downhill from there, and you entered old age with not many brain cells left.
What a joy it is to know that this belief is a load of rubbish.
Here is a great article, summarising recent research on the birth of new brain cells during our lives.
It is a reminder that you use your brain or you lose it, so lifelong learning stimulates the birth of new brain cells.
I can recommend learning a musical instrument – you can do it at any age. Making music on your own instrument really makes your brain work, whether you are a beginner or an old hand at it.