You cannot grow new brain cells
The old claim turned out to be wrong and was rejected by evidence.
TaughtYou are born with all the neurons you will ever have. Every one you lose is gone forever, so do not hit your head.
NowAdult neurogenesis occurs in the hippocampus, demonstrated in humans in 1998. The extent in adults remains genuinely contested, but the absolute claim is dead.
What actually happened
The dogma came from Santiago Ramón y Cajal, the founder of modern neuroscience, who wrote in 1928 that in adult brains the nerve paths are fixed and immutable. Coming from him, it settled the question for seventy years.
Joseph Altman reported new neurons in adult rats in the 1960s and was largely ignored. Elizabeth Gould's work on primates in the 1990s reopened it, and in 1998 Peter Eriksson and Fred Gage found new neurons in the hippocampi of deceased cancer patients who had been injected with a labeling compound during treatment, which marked cells that divided while they were alive. This is a fact where honesty requires flagging live controversy: a 2018 Nature paper found adult neurogenesis in humans drops to undetectable levels, while a paper in Cell Stem Cell the same year found the opposite. The safe statement is that the old absolute prohibition was wrong, and how much regeneration happens in an adult human brain is still an open research question.
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Who was taught this
Still standard through 2002, so anyone who finished school between 1950 and 1998 learned the earlier version.