I am not sure one can really say that there is an age where a mathematician peek. They do generally make their great works young, but that if often because it takes many, many years for the world to realize that the work they are making is a big deal (solving famous, difficult problems are one exception). In addition at some point they are going to end up more and more involved with the politics side of mathematics (organizing and presenting research, holding conferences, getting grants, etc) which means there is less and less time to do actual research.