Some people reach their 80s with memories sharper than many 50-year-olds. Scientists now think they know why: Their brains never stopped growing new cells. Scientists studying a rare group of older people known as superagers—those aged 80 and over whose memory rivals someone 30 years younger—have found that their brains produce new neurons at twice the rate of typical older adults. “For most of the last century, the prevailing belief was that brain cells only die as you age—you were born with what you had, and that was that,” Jordan Weiss, professor at the Optimal Aging Institute at NYU Grossman...