Layered

 We can think of intellectual life as layered in chronological stages over one's life time.  Of course, it is perfectly possible that someone can die young, and yet make a significant contribution at an early age. In those cases, we don't know what the later stages of development might have brought. 

One layer sits over another layer. We still know what we learned as children in school. There is a layer, then, of more specialized (and general) knowledge gained in college. 

Graduate school is layered on that. Now knowledge is even more specialized, but general knowledge should increase there as well.  

Early years as a professor, young researcher.  Then the "mid-career" scholar. Then the recognized expert at the highest rank.  All throughout these stages there are plateaux, and even possibly declines.  You can actually get worse as scholar, or become simply repetitive.  I know at one point I was very repetitive. I probably still am repetitive.  

Intellectual depth comes from the layering of these stages. Growth doesn't have to stop unless there is an actual cognitive decline.  Otherwise, we can be innovative as long as we want to.