Aside from 25 being a round number, it is believed that insurance companies chose 25 because the human brain isn’t fully developed until about that age. In fact, the frontal lobe – the part of the brain responsible for inhibiting our risk taking behavior and controlling our impulses – is one of the last parts of the human brain to fully mature. In males, the frontal lobes mature around the age of 24. Not surprising, the frontal lobes in women develop earlier – around the age of 23. Therefore, people under the age of 25 are more prone to risky behavior and are subject to higher auto insurance premiums compared to those over the age of 25.