People continue to mature and grow as individuals throughout our lifetimes (at least ideally). So the 45 year old you would probably not only be better equipped to make decisions than the 25 year old version, but also in all likelihood be a very different person, both literally (in the replacing of your body’s physical makeup in that gap) and mentally. Nevertheless, the 45 year old version of you and the much less mature and less accomplished 25 year old version are, in most societies, living under the exact same set of rules and restrictions. So why is some arbitrary number like 18 or 21 considered the cutoff point when society says everyone should be operating on a level playing field, legally speaking? What is it that makes a person an “adult” anyway? Is it the ability to drive a car without supervision? Consent to intimate relations? Ability to cast a vote? Serve in the military? Buy a beer? And at what age is a person really ready to handle these responsibilities? These are all difficult questions that don’t lend themselves to easy answers.
For example, at just 16 years old, Alexander the Great was busy conquering Maedi, when they dared revolt against Macedonia; also at 16, a peasant girl by the name of Jeanne d’Arc was taking her first steps into historical prominence by having the gall to approach a garrison commander to tell him how to do his job. At 15, one Charles Algernon Parsons was busy inventing the precursor to the modern automobile. At 16, Julius Caesar was heading his family after his father’s death. History is littered with individuals accomplishing remarkable “adult” things all below the age most countries would today say that they were sufficiently mature enough to be considered an adult. Unsurprisingly, for parts of history, the issue of when someone was ready to take over various adult activities largely fell to their family to decide.
Nevertheless, while maturity levels vary greatly from person to person and the people who know the individual best may be better able to determine things for everyday activities, a number that generally fits everyone is often needed to set certain communal laws. Thus, the age when a person is considered sufficiently able to manage these adult activities, and answer for others, is typically set by civil policy makers within a nation, and in certain cases, sometimes by each individual state within a country. For example, in California the age of consent is set at 18, a number largely proliferated by Hollywood and thus often considered the set age of consent in the United States. However, most states actually set it lower, and in many cases without restriction on age gap between the individuals after that cutoff; the important thing is simply consent.