someone at 24 has several more years of experience in the adult world. someone at 24 has several more years of neurological development (which isn't complete until around 25). in other words, at 24 someone has better context for decision-making and better decision-making ability than someone who is 18.
indeed, lots of people make poor decisions regardless of age. but statistically speaking, 24 year olds have resources (experience, development) which increase their capacity to make better decisions.