This Adult Month is a much more interesting experiment that I had anticipated. It's a curious thing to give yourself a basic question to apply to all your decisions, in this case, "What is most adult in this circumstance." So here are some preliminary thoughts.
First, Adulthood is about not grasping. Adyashanti has described the process by which people pick up and hold energy that comes from outside them, others anger, depression, whatnot. And he talks about a visualization one can do where you actually imagine both the front of your body being open--this is open heartedness, empathy--as well as the back of you body being open. The effect is that you are both feel the world and people around you, but you don't take on any of the energy (information) that when it comes to you, allows you to feel. You know but don't hold.
I feel most adult when I'm both most in open and most non-grasping. Awakeness seems to be a quality of adulthood, a willingness and readiness to remain in contact with whatever is happening in your life, without contracting in. You see, acknowledge, and accept. You don't pout, refuse, or deny.
And when there is call to act, you do so with a compassionate take on the lay of the land, and move forward without apology for your need or desire, while also taking responsibility for the consequences of your actions.
An Adult does not deny responsibility. And also, an Adult does not deny their own emotional experience in the process of taking responsibility; you don't stiffen in acceptance of your actions, you soften. If a tough decision is to be made, one neither denies ones own emotions, nor allows one's emotional reactions to overly determine the action.
An Adult accepts their fundamental lack of control and lets go of making control a primary motivator of action. Truly mature people hold their sense of Self lightly and ironically. It's there, it's active, but it's not very important.