Dr. Shruti Gohil, associate medical director of hospital epidemiology and assistant professor of infectious diseases with the School of Medicine at UC Irvine in Irvine, California, witnesses that feeling of invulnerability each year from younger people who don’t get the flu shot, even though the flu can and has killed young people. Brain development also plays a role. The prefrontal cortex in our brains, which affects judgment and impulse control, is the last part of the brain to develop, Gohil says. It typically develops around age 25. It’s one reason why young people tend to engage in riskier behavior.