That used to be the dominant theory - there is now evidence that new cells continue to grow - just the replacement rate slows down as we age. However we still d

That used to be the dominant theory - there is now evidence that new cells continue to grow - just the replacement rate slows down as we age. However we still do not really understand the mechanisms of neurogenesis (growth of new brain cells), the interplay with neural stem cells, cellular development dependencies or the pathological mechanisms associated with various diseases such as Alzheimer's. There continue to be new theories, emergent lab work etc but we still have a long long way to go before we have a clear insightful model of the mechanisms involved, never mind a predictive understanding for systemic outcomes.