There is increasing evidence that adult humans continue to grow new neurons in their brains , even up to the age of 100 years  and presumably beyond. Neurogenesis, or growth of new neurons, has been studied in adult rats and other mammals [e.g., 3]. Evidence of adult neurogenesis overturned the prevailing belief that the adult brain was fixed and incapable of regeneration. As these findings become more popularized, what will this mean for our concepts of aging?
It was long believed that brain development was completed around the age of 6 years old. This had disastrous consequences for people born with phenylketonuria (PKU) an inborn error of metabolism in which people cannot break down the amino acid phenylalanine. Phenylalanine is a neurotoxin that readily crosses the blood-brain barrier. What is remarkable is that PKU can be completely controlled by a very restrictive diet limited in phenylalinine. Children with PKU who are untreated will have significant cognitive deficits and have to be institutionalized as adults. Since the 1960s every baby in North America has been tested at birth for PKU.
Until the 1980s the standard recommendation was to discontinue treatment of children with PKU after the age of 6 years old. Subsequently researchers found a severe decline in intellectual function in children who had gone off diet . The current recommendation for people with PKU is that they stay on diet their entire lives.
Assumptions regarding the age at which the brain is “complete” has had consequences for people living with PKU. As the evidence of neurogenesis in adult humans emerges, how will that affect the way we think about age?
Given that people continue to learn and grow through their lives it actually seems a bit preposterous to believe that brain development would ever be complete. Setting aside neurogenesis for a moment, there seems to be a trend among people who are anxious about cognitive decline to engage in more cognitive exercises (e.g., sudoku, crossword puzzles). There is a “use it or lose it” mentality about cognition. The idea of neurogenesis is radical in the face of the belief that aging is essentially and only a process of deterioriation.
Ironically, it is not cognitive exercise that promotes neurogenesis, but rather physical exercise [5, 6]. I am starting to develop a different image of healthy aging. Rather than picturing the slow cognitive decline that seniors try to hold off through mind-exercises, I imagine engaged elderly people who are physically active. I think of walking groups (for those who are able) , or water exercises, or dance classes. Anything to get the heart pumping. What’s good for the heart, they say, is good for the brain.
Will we start to think of the elderly as more capable (old dogs, new tricks)? Will the idea of neurogenesis provide hope for those experiencing cognitive decline? Ultimately, how might long-term care facilities adjust their programming to accommodate the developing brains of their octogenarian residents?
 Eriksson PS et al. (1998). “Neurogenesis in the adult human hippocampus” Nature Medicine 4, 1313 – 1317 doi:10.1038/3305
 Knoth R, Singec I, Ditter M, Pantazis G, Capetian P, et al. (2010). “Murine Features of Neurogenesis in the Human Hippocampus across the Lifespan from 0 to 100 Years”. PLoS ONE 5(1): e8809. doi:10.1371/journal.pone.0008809
 Reynolds, BA; Weiss (1992). “Generation of neurons and astrocytes from isolated cells of the adult mammalian central nervous system”. Science (New York, N.Y.) 255 (5052): 1707–10. doi:10.1126/science.1553558
 Seashore, MR et al. (1985). “Loss of intellectual function in children with phenylketonuria after relaxation of dietary phenylalanine restriction”. Pediatrics 75(2): 226-32.
 Pereira AC et al. (2007) “An in vivo correlate of exercise-induced neurogenesis in the adult dentate gyrus” Proc Natl Acad Sci U S A. 104(13): 5638–5643. doi: 10.1073/pnas.0611721104