For all the practical importance of adulthood, the developmental connotations of the word are impoverished. As currently implemented, adulthood is an accident of survival. It occurs upon the attainment of our majority, usually at a legislated age.
Adults are exposed to more stringent standards of legal accountability than are minors. Along with this goes a certain privilege (so-called): adults may enter more freely into experiences that may expose the participants to harm. They may drink, and then drive. They may possess firearms. They may buy firearms for their children. They may even participate in selecting a government (wink). In contract law, this includes, to a certain extent, abrogation of our rights to hold others responsible for their mistreatment of us. (This is manifested as "fine print": legal gobble-de-gook that doesn't mean anything unless one knows which terms to invoke in a particular judicial context.)
From this, we might wonder whether the epithet "adult" actually confers a meaningful judgment on our personal qualities. This is explicit, in some associations. Standards for professional accreditation demonstrate that adulthood is not a universal qualification. Simply attaining our majority does not make us qualified to misrepresent our skills in a way that would cause others to abrogate their rights to hold us accountable for our errors. It also does not enable us to successfully perform brain surgery, or even bob a 'doo.
Perhaps most maddening, attaining our majority certainly doesn't make us sufficiently sophisticated psychologists to realize when we are giving power to those in whose hands it will be dangerous. In other words, we aren't certain to avoid making errors of judgment in the allocation of our love. Men tend to push too hard, and dishearten the people they are coaching to success. Women tend to be too soft, granting approval and support to the point that the loved one is capable of disengaging from reality.
If the privileges of adulthood are constrained to a specific context, then it stands to reason that we might successfully represent ourselves as adults in one context, and not in another. In that case, much of what we mean by "adult" is encompassed by the following: an adult understands power and love, and has the wisdom and experience to know when to apply both. In one context (i.e. - medical practice), we may understand the levers of power, and pass for an adult. In another (i.e. - legislative procedures), we may not.
We are never perfect adults. The standard calls us to hew to the goal of improvement for as long as we live.
Achieving mature adulthood is a never-ending project. Even if we succeed in attainment in one context, when we return to that ten years later, it would be surprising if the community had not advanced - or at the least modified - its standards and practices in the interim. For that reason, the progression to mature adulthood is the primary test and testament of will. To become truly accomplished, in the sense described here, requires that we engage unflinchingly and without reservation in learning to perceive and control the reality we choose to inhabit.