Some of the things that make you an adult are behavioral (mastery of basic manners, etiquette and hygiene), but the bedrock of adulthood stems from your thoughts and attitudes about life. Here's a short list of traits shared by all true adults.
Adults are responsible.Children are irresponsible. Rarely does anyone expect them to be anything else. But responsibility is by far the most important trait of an adult -- in fact, it's the trait that governs just about every other trait on this list. Being responsible is more than just claiming responsibility. It means you can be counted on to do what you promise; likewise, if you know you can't do it, you don't make promises you can't keep. It means you set and keep appointments. It means you perform to the best of your ability. And it means that when things go wrong -- and things often do go wrong -- you don't start looking around for someone else to blame. You take your share of the responsibility and your share of the consequences.
Adults are honest.Children tend to lie, especially if they think it will get them out of trouble. But you're an adult, and have learned to be honest, which is more than just being truthful. Honesty means doing what you say you'll do, rather than making people momentarily happy and ultimately disappointed by giving them a promise you know you won't keep. Honesty means paying your bills on time and in full. It means honoring your contracts. It means not taking things that aren't yours. And sometimes it means telling people what they need to hear, rather than what they want to hear.
Adults are effective.Children often wish for a genie, a windfall or a lucky break to change their lives for good; you might sometimes fantasize about these things too, but you don't wait around for them to happen. When you're an adult, you recognize that you are the most powerful agent of change in your own life, and act accordingly. When things break, you don't wait around for someone else to take care of the problem; you figure out what you have to do to get them fixed. When projects need to be done, you do them (or you hire a responsible person to do them, and pay a fair wage). And when you want your life to change, you figure out what you want, determine where you currently are, and develop a plan to get you from here to there. Then you put the plan into action. Such plans often require consistent, hard work, which leads us to the next trait...
Adults are hard workers.Children don't usually know how to work; they just expect other people to take care of them. Like all adults, you have a strong work ethic. You know how to work well and effectively. You can be justifiably proud of your ability to provide for yourself and your family. You can do hard things; in fact, you deliberately give yourself challenges. You have learned how to bring more value to a company than you take from it. You don't pass up chances to learn and improve because you want to be really good at what you do. And you can take constructive criticism.
Adults are thoughtful.Most children are so obsessed with themselves and their own concerns that it never occurs to them to consider anyone else. As an adult, you've learned to be thoughtful of other people and their needs. To that end, you've learned the basics of hygiene, manners and etiquette (a way of being kind to people even if you don't know them). You make it a habit to look around for others in need of assistance, and give it when possible. You show compassion to the poor and do what you can, donating money and time, to give them a hand up. You give up your seat on the train to the old, the infirm, and pregnant women. You remember to smile and be patient with people who seem to be having a hard day. You give thoughtful gifts as well as receiving them. When others are kind enough to do things for you, you recognize their effort, remember to thank them and do them a kindness as well in future.
Adults are capable of sacrifice.Children want whatever interests them, and they want it now; they're not willing to wait for even a few minutes to get the object of their desire. As an adult, you often have strong desires too, but you've learned the nature of sacrifice: giving up something you want now for something you want more. The only things you buy on credit are an education, a house, and (maybe) a car. You save up to buy other things only as you can afford them. Since you've realized that things aren't all that important to happiness, you don't feel the urge to "keep up" with everyone else, purchasing only those things that are truly useful and/or meaningful to you. You don't eat that donut today because you want to be slim and healthy in several months. You work to get a good education in order to get greater enjoyment out of life, and to increase your opportunities as well.
There's a reason why society urges people to wait until they're adults before they become parents: raising a child requires multiple sacrifices. The first, if you're a woman, is the sacrifice of your youthful body to the changes that come with pregnancy and childbirth. Then you sacrifice hours of sleep and your sense of privacy as the child needs to be fed; you sacrifice your squeamishness whenever you change a diaper; you sacrifice your free time to take the child places; you sacrifice your impatience to teach the child life skills; you sacrifice your disposable income to buy things the child needs (everything from baby food to college tuition); you sacrifice your need to be popular when that child, angry at an act of discipline, screams at you more than once, "I hate you! I wish you were dead!" But sacrifice means giving up something you want now for something you want more. What you want more than anything is for that child to grow up and become a responsible, honest, effective, hard-working, thoughtful, sacrificing, loving adult.
In other words, someone a little bit like you.