I want to point to two recent examples of studies on brain renewal in aging humans and other animals. First, a Stanford group finds that the normal decay in the effectiveness of the microglial brain cells that clean up brain garbage (protein deposits and cellular debris) correlates with the increased appearance of a single gene product, the B cell receptor protein CD22. They find that "Long-term central nervous system delivery of an antibody that blocks CD22 function reprograms microglia towards a homeostatic transcriptional state and improves cognitive function in aged mice."
Second, Zimmer does a nice review of work on the brain protein Klotho. Mice bred to make extra Klotho live 30 percent longer, and protects those with symptoms of Alzheimer's disease from cognitive decline. Klotho not only protects, but enhanced brain function, even in young mice. A recent study suggests that Klotho may also provide some protection from Alzheimer’s disease to people as well.